:mod:`logging` --- Logging facility for Python ============================================== .. module:: logging :synopsis: Flexible error logging system for applications. .. moduleauthor:: Vinay Sajip .. sectionauthor:: Vinay Sajip .. index:: pair: Errors; logging This module defines functions and classes which implement a flexible error logging system for applications. Logging is performed by calling methods on instances of the :class:`Logger` class (hereafter called :dfn:`loggers`). Each instance has a name, and they are conceptually arranged in a namespace hierarchy using dots (periods) as separators. For example, a logger named "scan" is the parent of loggers "scan.text", "scan.html" and "scan.pdf". Logger names can be anything you want, and indicate the area of an application in which a logged message originates. Logged messages also have levels of importance associated with them. The default levels provided are :const:`DEBUG`, :const:`INFO`, :const:`WARNING`, :const:`ERROR` and :const:`CRITICAL`. As a convenience, you indicate the importance of a logged message by calling an appropriate method of :class:`Logger`. The methods are :meth:`debug`, :meth:`info`, :meth:`warning`, :meth:`error` and :meth:`critical`, which mirror the default levels. You are not constrained to use these levels: you can specify your own and use a more general :class:`Logger` method, :meth:`log`, which takes an explicit level argument. Logging tutorial ---------------- The key benefit of having the logging API provided by a standard library module is that all Python modules can participate in logging, so your application log can include messages from third-party modules. It is, of course, possible to log messages with different verbosity levels or to different destinations. Support for writing log messages to files, HTTP GET/POST locations, email via SMTP, generic sockets, or OS-specific logging mechanisms are all supported by the standard module. You can also create your own log destination class if you have special requirements not met by any of the built-in classes. Simple examples ^^^^^^^^^^^^^^^ .. sectionauthor:: Doug Hellmann .. (see ) Most applications are probably going to want to log to a file, so let's start with that case. Using the :func:`basicConfig` function, we can set up the default handler so that debug messages are written to a file (in the example, we assume that you have the appropriate permissions to create a file called *example.log* in the current directory):: import logging LOG_FILENAME = 'example.log' logging.basicConfig(filename=LOG_FILENAME,level=logging.DEBUG) logging.debug('This message should go to the log file') And now if we open the file and look at what we have, we should find the log message:: DEBUG:root:This message should go to the log file If you run the script repeatedly, the additional log messages are appended to the file. To create a new file each time, you can pass a *filemode* argument to :func:`basicConfig` with a value of ``'w'``. Rather than managing the file size yourself, though, it is simpler to use a :class:`RotatingFileHandler`:: import glob import logging import logging.handlers LOG_FILENAME = 'logging_rotatingfile_example.out' # Set up a specific logger with our desired output level my_logger = logging.getLogger('MyLogger') my_logger.setLevel(logging.DEBUG) # Add the log message handler to the logger handler = logging.handlers.RotatingFileHandler( LOG_FILENAME, maxBytes=20, backupCount=5) my_logger.addHandler(handler) # Log some messages for i in range(20): my_logger.debug('i = %d' % i) # See what files are created logfiles = glob.glob('%s*' % LOG_FILENAME) for filename in logfiles: print(filename) The result should be 6 separate files, each with part of the log history for the application:: logging_rotatingfile_example.out logging_rotatingfile_example.out.1 logging_rotatingfile_example.out.2 logging_rotatingfile_example.out.3 logging_rotatingfile_example.out.4 logging_rotatingfile_example.out.5 The most current file is always :file:`logging_rotatingfile_example.out`, and each time it reaches the size limit it is renamed with the suffix ``.1``. Each of the existing backup files is renamed to increment the suffix (``.1`` becomes ``.2``, etc.) and the ``.6`` file is erased. Obviously this example sets the log length much much too small as an extreme example. You would want to set *maxBytes* to an appropriate value. Another useful feature of the logging API is the ability to produce different messages at different log levels. This allows you to instrument your code with debug messages, for example, but turning the log level down so that those debug messages are not written for your production system. The default levels are ``NOTSET``, ``DEBUG``, ``INFO``, ``WARNING``, ``ERROR`` and ``CRITICAL``. The logger, handler, and log message call each specify a level. The log message is only emitted if the handler and logger are configured to emit messages of that level or lower. For example, if a message is ``CRITICAL``, and the logger is set to ``ERROR``, the message is emitted. If a message is a ``WARNING``, and the logger is set to produce only ``ERROR``\s, the message is not emitted:: import logging import sys LEVELS = {'debug': logging.DEBUG, 'info': logging.INFO, 'warning': logging.WARNING, 'error': logging.ERROR, 'critical': logging.CRITICAL} if len(sys.argv) > 1: level_name = sys.argv[1] level = LEVELS.get(level_name, logging.NOTSET) logging.basicConfig(level=level) logging.debug('This is a debug message') logging.info('This is an info message') logging.warning('This is a warning message') logging.error('This is an error message') logging.critical('This is a critical error message') Run the script with an argument like 'debug' or 'warning' to see which messages show up at different levels:: $ python logging_level_example.py debug DEBUG:root:This is a debug message INFO:root:This is an info message WARNING:root:This is a warning message ERROR:root:This is an error message CRITICAL:root:This is a critical error message $ python logging_level_example.py info INFO:root:This is an info message WARNING:root:This is a warning message ERROR:root:This is an error message CRITICAL:root:This is a critical error message You will notice that these log messages all have ``root`` embedded in them. The logging module supports a hierarchy of loggers with different names. An easy way to tell where a specific log message comes from is to use a separate logger object for each of your modules. Each new logger "inherits" the configuration of its parent, and log messages sent to a logger include the name of that logger. Optionally, each logger can be configured differently, so that messages from different modules are handled in different ways. Let's look at a simple example of how to log from different modules so it is easy to trace the source of the message:: import logging logging.basicConfig(level=logging.WARNING) logger1 = logging.getLogger('package1.module1') logger2 = logging.getLogger('package2.module2') logger1.warning('This message comes from one module') logger2.warning('And this message comes from another module') And the output:: $ python logging_modules_example.py WARNING:package1.module1:This message comes from one module WARNING:package2.module2:And this message comes from another module There are many more options for configuring logging, including different log message formatting options, having messages delivered to multiple destinations, and changing the configuration of a long-running application on the fly using a socket interface. All of these options are covered in depth in the library module documentation. Loggers ^^^^^^^ The logging library takes a modular approach and offers the several categories of components: loggers, handlers, filters, and formatters. Loggers expose the interface that application code directly uses. Handlers send the log records to the appropriate destination. Filters provide a finer grained facility for determining which log records to send on to a handler. Formatters specify the layout of the resultant log record. :class:`Logger` objects have a threefold job. First, they expose several methods to application code so that applications can log messages at runtime. Second, logger objects determine which log messages to act upon based upon severity (the default filtering facility) or filter objects. Third, logger objects pass along relevant log messages to all interested log handlers. The most widely used methods on logger objects fall into two categories: configuration and message sending. * :meth:`Logger.setLevel` specifies the lowest-severity log message a logger will handle, where debug is the lowest built-in severity level and critical is the highest built-in severity. For example, if the severity level is info, the logger will handle only info, warning, error, and critical messages and will ignore debug messages. * :meth:`Logger.addFilter` and :meth:`Logger.removeFilter` add and remove filter objects from the logger object. This tutorial does not address filters. With the logger object configured, the following methods create log messages: * :meth:`Logger.debug`, :meth:`Logger.info`, :meth:`Logger.warning`, :meth:`Logger.error`, and :meth:`Logger.critical` all create log records with a message and a level that corresponds to their respective method names. The message is actually a format string, which may contain the standard string substitution syntax of :const:`%s`, :const:`%d`, :const:`%f`, and so on. The rest of their arguments is a list of objects that correspond with the substitution fields in the message. With regard to :const:`**kwargs`, the logging methods care only about a keyword of :const:`exc_info` and use it to determine whether to log exception information. * :meth:`Logger.exception` creates a log message similar to :meth:`Logger.error`. The difference is that :meth:`Logger.exception` dumps a stack trace along with it. Call this method only from an exception handler. * :meth:`Logger.log` takes a log level as an explicit argument. This is a little more verbose for logging messages than using the log level convenience methods listed above, but this is how to log at custom log levels. :func:`getLogger` returns a reference to a logger instance with the specified name if it is provided, or ``root`` if not. The names are period-separated hierarchical structures. Multiple calls to :func:`getLogger` with the same name will return a reference to the same logger object. Loggers that are further down in the hierarchical list are children of loggers higher up in the list. For example, given a logger with a name of ``foo``, loggers with names of ``foo.bar``, ``foo.bar.baz``, and ``foo.bam`` are all descendants of ``foo``. Child loggers propagate messages up to the handlers associated with their ancestor loggers. Because of this, it is unnecessary to define and configure handlers for all the loggers an application uses. It is sufficient to configure handlers for a top-level logger and create child loggers as needed. Handlers ^^^^^^^^ :class:`Handler` objects are responsible for dispatching the appropriate log messages (based on the log messages' severity) to the handler's specified destination. Logger objects can add zero or more handler objects to themselves with an :func:`addHandler` method. As an example scenario, an application may want to send all log messages to a log file, all log messages of error or higher to stdout, and all messages of critical to an email address. This scenario requires three individual handlers where each handler is responsible for sending messages of a specific severity to a specific location. The standard library includes quite a few handler types; this tutorial uses only :class:`StreamHandler` and :class:`FileHandler` in its examples. There are very few methods in a handler for application developers to concern themselves with. The only handler methods that seem relevant for application developers who are using the built-in handler objects (that is, not creating custom handlers) are the following configuration methods: * The :meth:`Handler.setLevel` method, just as in logger objects, specifies the lowest severity that will be dispatched to the appropriate destination. Why are there two :func:`setLevel` methods? The level set in the logger determines which severity of messages it will pass to its handlers. The level set in each handler determines which messages that handler will send on. * :func:`setFormatter` selects a Formatter object for this handler to use. * :func:`addFilter` and :func:`removeFilter` respectively configure and deconfigure filter objects on handlers. Application code should not directly instantiate and use instances of :class:`Handler`. Instead, the :class:`Handler` class is a base class that defines the interface that all handlers should have and establishes some default behavior that child classes can use (or override). Formatters ^^^^^^^^^^ Formatter objects configure the final order, structure, and contents of the log message. Unlike the base :class:`logging.Handler` class, application code may instantiate formatter classes, although you could likely subclass the formatter if your application needs special behavior. The constructor takes three optional arguments -- a message format string, a date format string and a style indicator. .. method:: logging.Formatter.__init__(fmt=None, datefmt=None, style='%') If there is no message format string, the default is to use the raw message. If there is no date format string, the default date format is:: %Y-%m-%d %H:%M:%S with the milliseconds tacked on at the end. The ``style`` is one of `%`, '{' or '$'. If one of these is not specified, then '%' will be used. If the ``style`` is '%', the message format string uses ``%()s`` styled string substitution; the possible keys are documented in :ref:`formatter-objects`. If the style is '{', the message format string is assumed to be compatible with :meth:`str.format` (using keyword arguments), while if the style is '$' then the message format string should conform to what is expected by :meth:`string.Template.substitute`. .. versionchanged:: 3.2 Added the ``style`` parameter. The following message format string will log the time in a human-readable format, the severity of the message, and the contents of the message, in that order:: "%(asctime)s - %(levelname)s - %(message)s" Formatters use a user-configurable function to convert the creation time of a record to a tuple. By default, :func:`time.localtime` is used; to change this for a particular formatter instance, set the ``converter`` attribute of the instance to a function with the same signature as :func:`time.localtime` or :func:`time.gmtime`. To change it for all formatters, for example if you want all logging times to be shown in GMT, set the ``converter`` attribute in the Formatter class (to ``time.gmtime`` for GMT display). Configuring Logging ^^^^^^^^^^^^^^^^^^^ Programmers can configure logging in three ways: 1. Creating loggers, handlers, and formatters explicitly using Python code that calls the configuration methods listed above. 2. Creating a logging config file and reading it using the :func:`fileConfig` function. 3. Creating a dictionary of configuration information and passing it to the :func:`dictConfig` function. The following example configures a very simple logger, a console handler, and a simple formatter using Python code:: import logging # create logger logger = logging.getLogger("simple_example") logger.setLevel(logging.DEBUG) # create console handler and set level to debug ch = logging.StreamHandler() ch.setLevel(logging.DEBUG) # create formatter formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s") # add formatter to ch ch.setFormatter(formatter) # add ch to logger logger.addHandler(ch) # "application" code logger.debug("debug message") logger.info("info message") logger.warn("warn message") logger.error("error message") logger.critical("critical message") Running this module from the command line produces the following output:: $ python simple_logging_module.py 2005-03-19 15:10:26,618 - simple_example - DEBUG - debug message 2005-03-19 15:10:26,620 - simple_example - INFO - info message 2005-03-19 15:10:26,695 - simple_example - WARNING - warn message 2005-03-19 15:10:26,697 - simple_example - ERROR - error message 2005-03-19 15:10:26,773 - simple_example - CRITICAL - critical message The following Python module creates a logger, handler, and formatter nearly identical to those in the example listed above, with the only difference being the names of the objects:: import logging import logging.config logging.config.fileConfig("logging.conf") # create logger logger = logging.getLogger("simpleExample") # "application" code logger.debug("debug message") logger.info("info message") logger.warn("warn message") logger.error("error message") logger.critical("critical message") Here is the logging.conf file:: [loggers] keys=root,simpleExample [handlers] keys=consoleHandler [formatters] keys=simpleFormatter [logger_root] level=DEBUG handlers=consoleHandler [logger_simpleExample] level=DEBUG handlers=consoleHandler qualname=simpleExample propagate=0 [handler_consoleHandler] class=StreamHandler level=DEBUG formatter=simpleFormatter args=(sys.stdout,) [formatter_simpleFormatter] format=%(asctime)s - %(name)s - %(levelname)s - %(message)s datefmt= The output is nearly identical to that of the non-config-file-based example:: $ python simple_logging_config.py 2005-03-19 15:38:55,977 - simpleExample - DEBUG - debug message 2005-03-19 15:38:55,979 - simpleExample - INFO - info message 2005-03-19 15:38:56,054 - simpleExample - WARNING - warn message 2005-03-19 15:38:56,055 - simpleExample - ERROR - error message 2005-03-19 15:38:56,130 - simpleExample - CRITICAL - critical message You can see that the config file approach has a few advantages over the Python code approach, mainly separation of configuration and code and the ability of noncoders to easily modify the logging properties. Note that the class names referenced in config files need to be either relative to the logging module, or absolute values which can be resolved using normal import mechanisms. Thus, you could use either :class:`handlers.WatchedFileHandler` (relative to the logging module) or ``mypackage.mymodule.MyHandler`` (for a class defined in package ``mypackage`` and module ``mymodule``, where ``mypackage`` is available on the Python import path). In Python 3.2, a new means of configuring logging has been introduced, using dictionaries to hold configuration information. This provides a superset of the functionality of the config-file-based approach outlined above, and is the recommended configuration method for new applications and deployments. Because a Python dictionary is used to hold configuration information, and since you can populate that dictionary using different means, you have more options for configuration. For example, you can use a configuration file in JSON format, or, if you have access to YAML processing functionality, a file in YAML format, to populate the configuration dictionary. Or, of course, you can construct the dictionary in Python code, receive it in pickled form over a socket, or use whatever approach makes sense for your application. Here's an example of the same configuration as above, in YAML format for the new dictionary-based approach:: version: 1 formatters: simple: format: format=%(asctime)s - %(name)s - %(levelname)s - %(message)s handlers: console: class: logging.StreamHandler level: DEBUG formatter: simple stream: ext://sys.stdout loggers: simpleExample: level: DEBUG handlers: [console] propagate: no root: level: DEBUG handlers: [console] For more information about logging using a dictionary, see :ref:`logging-config-api`. .. _library-config: Configuring Logging for a Library ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ When developing a library which uses logging, some consideration needs to be given to its configuration. If the using application does not use logging, and library code makes logging calls, then a one-off message "No handlers could be found for logger X.Y.Z" is printed to the console. This message is intended to catch mistakes in logging configuration, but will confuse an application developer who is not aware of logging by the library. In addition to documenting how a library uses logging, a good way to configure library logging so that it does not cause a spurious message is to add a handler which does nothing. This avoids the message being printed, since a handler will be found: it just doesn't produce any output. If the library user configures logging for application use, presumably that configuration will add some handlers, and if levels are suitably configured then logging calls made in library code will send output to those handlers, as normal. A do-nothing handler can be simply defined as follows:: import logging class NullHandler(logging.Handler): def emit(self, record): pass An instance of this handler should be added to the top-level logger of the logging namespace used by the library. If all logging by a library *foo* is done using loggers with names matching "foo.x.y", then the code:: import logging h = NullHandler() logging.getLogger("foo").addHandler(h) should have the desired effect. If an organisation produces a number of libraries, then the logger name specified can be "orgname.foo" rather than just "foo". **PLEASE NOTE:** It is strongly advised that you *do not add any handlers other than* :class:`NullHandler` *to your library's loggers*. This is because the configuration of handlers is the prerogative of the application developer who uses your library. The application developer knows their target audience and what handlers are most appropriate for their application: if you add handlers "under the hood", you might well interfere with their ability to carry out unit tests and deliver logs which suit their requirements. .. versionadded:: 3.1 The :class:`NullHandler` class. Logging Levels -------------- The numeric values of logging levels are given in the following table. These are primarily of interest if you want to define your own levels, and need them to have specific values relative to the predefined levels. If you define a level with the same numeric value, it overwrites the predefined value; the predefined name is lost. +--------------+---------------+ | Level | Numeric value | +==============+===============+ | ``CRITICAL`` | 50 | +--------------+---------------+ | ``ERROR`` | 40 | +--------------+---------------+ | ``WARNING`` | 30 | +--------------+---------------+ | ``INFO`` | 20 | +--------------+---------------+ | ``DEBUG`` | 10 | +--------------+---------------+ | ``NOTSET`` | 0 | +--------------+---------------+ Levels can also be associated with loggers, being set either by the developer or through loading a saved logging configuration. When a logging method is called on a logger, the logger compares its own level with the level associated with the method call. If the logger's level is higher than the method call's, no logging message is actually generated. This is the basic mechanism controlling the verbosity of logging output. Logging messages are encoded as instances of the :class:`LogRecord` class. When a logger decides to actually log an event, a :class:`LogRecord` instance is created from the logging message. Logging messages are subjected to a dispatch mechanism through the use of :dfn:`handlers`, which are instances of subclasses of the :class:`Handler` class. Handlers are responsible for ensuring that a logged message (in the form of a :class:`LogRecord`) ends up in a particular location (or set of locations) which is useful for the target audience for that message (such as end users, support desk staff, system administrators, developers). Handlers are passed :class:`LogRecord` instances intended for particular destinations. Each logger can have zero, one or more handlers associated with it (via the :meth:`addHandler` method of :class:`Logger`). In addition to any handlers directly associated with a logger, *all handlers associated with all ancestors of the logger* are called to dispatch the message (unless the *propagate* flag for a logger is set to a false value, at which point the passing to ancestor handlers stops). Just as for loggers, handlers can have levels associated with them. A handler's level acts as a filter in the same way as a logger's level does. If a handler decides to actually dispatch an event, the :meth:`emit` method is used to send the message to its destination. Most user-defined subclasses of :class:`Handler` will need to override this :meth:`emit`. .. _custom-levels: Custom Levels ^^^^^^^^^^^^^ Defining your own levels is possible, but should not be necessary, as the existing levels have been chosen on the basis of practical experience. However, if you are convinced that you need custom levels, great care should be exercised when doing this, and it is possibly *a very bad idea to define custom levels if you are developing a library*. That's because if multiple library authors all define their own custom levels, there is a chance that the logging output from such multiple libraries used together will be difficult for the using developer to control and/or interpret, because a given numeric value might mean different things for different libraries. Useful Handlers --------------- In addition to the base :class:`Handler` class, many useful subclasses are provided: #. :class:`StreamHandler` instances send messages to streams (file-like objects). #. :class:`FileHandler` instances send messages to disk files. .. module:: logging.handlers #. :class:`BaseRotatingHandler` is the base class for handlers that rotate log files at a certain point. It is not meant to be instantiated directly. Instead, use :class:`RotatingFileHandler` or :class:`TimedRotatingFileHandler`. #. :class:`RotatingFileHandler` instances send messages to disk files, with support for maximum log file sizes and log file rotation. #. :class:`TimedRotatingFileHandler` instances send messages to disk files, rotating the log file at certain timed intervals. #. :class:`SocketHandler` instances send messages to TCP/IP sockets. #. :class:`DatagramHandler` instances send messages to UDP sockets. #. :class:`SMTPHandler` instances send messages to a designated email address. #. :class:`SysLogHandler` instances send messages to a Unix syslog daemon, possibly on a remote machine. #. :class:`NTEventLogHandler` instances send messages to a Windows NT/2000/XP event log. #. :class:`MemoryHandler` instances send messages to a buffer in memory, which is flushed whenever specific criteria are met. #. :class:`HTTPHandler` instances send messages to an HTTP server using either ``GET`` or ``POST`` semantics. #. :class:`WatchedFileHandler` instances watch the file they are logging to. If the file changes, it is closed and reopened using the file name. This handler is only useful on Unix-like systems; Windows does not support the underlying mechanism used. #. :class:`QueueHandler` instances send messages to a queue, such as those implemented in the :mod:`queue` or :mod:`multiprocessing` modules. .. currentmodule:: logging #. :class:`NullHandler` instances do nothing with error messages. They are used by library developers who want to use logging, but want to avoid the "No handlers could be found for logger XXX" message which can be displayed if the library user has not configured logging. See :ref:`library-config` for more information. .. versionadded:: 3.1 The :class:`NullHandler` class. .. versionadded:: 3.2 The :class:`QueueHandler` class. The :class:`NullHandler`, :class:`StreamHandler` and :class:`FileHandler` classes are defined in the core logging package. The other handlers are defined in a sub- module, :mod:`logging.handlers`. (There is also another sub-module, :mod:`logging.config`, for configuration functionality.) Logged messages are formatted for presentation through instances of the :class:`Formatter` class. They are initialized with a format string suitable for use with the % operator and a dictionary. For formatting multiple messages in a batch, instances of :class:`BufferingFormatter` can be used. In addition to the format string (which is applied to each message in the batch), there is provision for header and trailer format strings. When filtering based on logger level and/or handler level is not enough, instances of :class:`Filter` can be added to both :class:`Logger` and :class:`Handler` instances (through their :meth:`addFilter` method). Before deciding to process a message further, both loggers and handlers consult all their filters for permission. If any filter returns a false value, the message is not processed further. The basic :class:`Filter` functionality allows filtering by specific logger name. If this feature is used, messages sent to the named logger and its children are allowed through the filter, and all others dropped. Module-Level Functions ---------------------- In addition to the classes described above, there are a number of module- level functions. .. function:: getLogger(name=None) Return a logger with the specified name or, if name is ``None``, return a logger which is the root logger of the hierarchy. If specified, the name is typically a dot-separated hierarchical name like *"a"*, *"a.b"* or *"a.b.c.d"*. Choice of these names is entirely up to the developer who is using logging. All calls to this function with a given name return the same logger instance. This means that logger instances never need to be passed between different parts of an application. .. function:: getLoggerClass() Return either the standard :class:`Logger` class, or the last class passed to :func:`setLoggerClass`. This function may be called from within a new class definition, to ensure that installing a customised :class:`Logger` class will not undo customisations already applied by other code. For example:: class MyLogger(logging.getLoggerClass()): # ... override behaviour here .. function:: getLogRecordFactory() Return a callable which is used to create a :class:`LogRecord`. .. versionadded:: 3.2 This function has been provided, along with :func:`setLogRecordFactory`, to allow developers more control over how the :class:`LogRecord` representing a logging event is constructed. See :func:`setLogRecordFactory` for more information about the how the factory is called. .. function:: debug(msg, *args, **kwargs) Logs a message with level :const:`DEBUG` on the root logger. The *msg* is the message format string, and the *args* are the arguments which are merged into *msg* using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.) There are three keyword arguments in *kwargs* which are inspected: *exc_info* which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by :func:`sys.exc_info`) is provided, it is used; otherwise, :func:`sys.exc_info` is called to get the exception information. The second optional keyword argument is *stack_info*, which defaults to False. If specified as True, stack information is added to the logging message, including the actual logging call. Note that this is not the same stack information as that displayed through specifying *exc_info*: The former is stack frames from the bottom of the stack up to the logging call in the current thread, whereas the latter is information about stack frames which have been unwound, following an exception, while searching for exception handlers. You can specify *stack_info* independently of *exc_info*, e.g. to just show how you got to a certain point in your code, even when no exceptions were raised. The stack frames are printed following a header line which says:: Stack (most recent call last): This mimics the `Traceback (most recent call last):` which is used when displaying exception frames. The third optional keyword argument is *extra* which can be used to pass a dictionary which is used to populate the __dict__ of the LogRecord created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:: FORMAT = "%(asctime)-15s %(clientip)s %(user)-8s %(message)s" logging.basicConfig(format=FORMAT) d = {'clientip': '192.168.0.1', 'user': 'fbloggs'} logging.warning("Protocol problem: %s", "connection reset", extra=d) would print something like:: 2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset The keys in the dictionary passed in *extra* should not clash with the keys used by the logging system. (See the :class:`Formatter` documentation for more information on which keys are used by the logging system.) If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the :class:`Formatter` has been set up with a format string which expects 'clientip' and 'user' in the attribute dictionary of the LogRecord. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the *extra* dictionary with these keys. While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized :class:`Formatter`\ s would be used with particular :class:`Handler`\ s. .. versionadded:: 3.2 The *stack_info* parameter was added. .. function:: info(msg, *args, **kwargs) Logs a message with level :const:`INFO` on the root logger. The arguments are interpreted as for :func:`debug`. .. function:: warning(msg, *args, **kwargs) Logs a message with level :const:`WARNING` on the root logger. The arguments are interpreted as for :func:`debug`. .. function:: error(msg, *args, **kwargs) Logs a message with level :const:`ERROR` on the root logger. The arguments are interpreted as for :func:`debug`. .. function:: critical(msg, *args, **kwargs) Logs a message with level :const:`CRITICAL` on the root logger. The arguments are interpreted as for :func:`debug`. .. function:: exception(msg, *args) Logs a message with level :const:`ERROR` on the root logger. The arguments are interpreted as for :func:`debug`. Exception info is added to the logging message. This function should only be called from an exception handler. .. function:: log(level, msg, *args, **kwargs) Logs a message with level *level* on the root logger. The other arguments are interpreted as for :func:`debug`. PLEASE NOTE: The above module-level functions which delegate to the root logger should *not* be used in threads, in versions of Python earlier than 2.7.1 and 3.2, unless at least one handler has been added to the root logger *before* the threads are started. These convenience functions call :func:`basicConfig` to ensure that at least one handler is available; in earlier versions of Python, this can (under rare circumstances) lead to handlers being added multiple times to the root logger, which can in turn lead to multiple messages for the same event. .. function:: disable(lvl) Provides an overriding level *lvl* for all loggers which takes precedence over the logger's own level. When the need arises to temporarily throttle logging output down across the whole application, this function can be useful. Its effect is to disable all logging calls of severity *lvl* and below, so that if you call it with a value of INFO, then all INFO and DEBUG events would be discarded, whereas those of severity WARNING and above would be processed according to the logger's effective level. .. function:: addLevelName(lvl, levelName) Associates level *lvl* with text *levelName* in an internal dictionary, which is used to map numeric levels to a textual representation, for example when a :class:`Formatter` formats a message. This function can also be used to define your own levels. The only constraints are that all levels used must be registered using this function, levels should be positive integers and they should increase in increasing order of severity. NOTE: If you are thinking of defining your own levels, please see the section on :ref:`custom-levels`. .. function:: getLevelName(lvl) Returns the textual representation of logging level *lvl*. If the level is one of the predefined levels :const:`CRITICAL`, :const:`ERROR`, :const:`WARNING`, :const:`INFO` or :const:`DEBUG` then you get the corresponding string. If you have associated levels with names using :func:`addLevelName` then the name you have associated with *lvl* is returned. If a numeric value corresponding to one of the defined levels is passed in, the corresponding string representation is returned. Otherwise, the string "Level %s" % lvl is returned. .. function:: makeLogRecord(attrdict) Creates and returns a new :class:`LogRecord` instance whose attributes are defined by *attrdict*. This function is useful for taking a pickled :class:`LogRecord` attribute dictionary, sent over a socket, and reconstituting it as a :class:`LogRecord` instance at the receiving end. .. function:: basicConfig(**kwargs) Does basic configuration for the logging system by creating a :class:`StreamHandler` with a default :class:`Formatter` and adding it to the root logger. The functions :func:`debug`, :func:`info`, :func:`warning`, :func:`error` and :func:`critical` will call :func:`basicConfig` automatically if no handlers are defined for the root logger. This function does nothing if the root logger already has handlers configured for it. PLEASE NOTE: This function should be called from the main thread before other threads are started. In versions of Python prior to 2.7.1 and 3.2, if this function is called from multiple threads, it is possible (in rare circumstances) that a handler will be added to the root logger more than once, leading to unexpected results such as messages being duplicated in the log. The following keyword arguments are supported. +--------------+---------------------------------------------+ | Format | Description | +==============+=============================================+ | ``filename`` | Specifies that a FileHandler be created, | | | using the specified filename, rather than a | | | StreamHandler. | +--------------+---------------------------------------------+ | ``filemode`` | Specifies the mode to open the file, if | | | filename is specified (if filemode is | | | unspecified, it defaults to 'a'). | +--------------+---------------------------------------------+ | ``format`` | Use the specified format string for the | | | handler. | +--------------+---------------------------------------------+ | ``datefmt`` | Use the specified date/time format. | +--------------+---------------------------------------------+ | ``style`` | If ``format`` is specified, use this style | | | for the format string. One of '%', '{' or | | | '$' for %-formatting, :meth:`str.format` or | | | :class:`string.Template` respectively, and | | | defaulting to '%' if not specified. | +--------------+---------------------------------------------+ | ``level`` | Set the root logger level to the specified | | | level. | +--------------+---------------------------------------------+ | ``stream`` | Use the specified stream to initialize the | | | StreamHandler. Note that this argument is | | | incompatible with 'filename' - if both are | | | present, 'stream' is ignored. | +--------------+---------------------------------------------+ .. versionchanged:: 3.2 The ``style`` argument was added. .. function:: shutdown() Informs the logging system to perform an orderly shutdown by flushing and closing all handlers. This should be called at application exit and no further use of the logging system should be made after this call. .. function:: setLoggerClass(klass) Tells the logging system to use the class *klass* when instantiating a logger. The class should define :meth:`__init__` such that only a name argument is required, and the :meth:`__init__` should call :meth:`Logger.__init__`. This function is typically called before any loggers are instantiated by applications which need to use custom logger behavior. .. function:: setLogRecordFactory(factory) Set a callable which is used to create a :class:`LogRecord`. :param factory: The factory callable to be used to instantiate a log record. .. versionadded:: 3.2 This function has been provided, along with :func:`getLogRecordFactory`, to allow developers more control over how the :class:`LogRecord` representing a logging event is constructed. The factory has the following signature: ``factory(name, level, fn, lno, msg, args, exc_info, func=None, sinfo=None, \*\*kwargs)`` :name: The logger name. :level: The logging level (numeric). :fn: The full pathname of the file where the logging call was made. :lno: The line number in the file where the logging call was made. :msg: The logging message. :args: The arguments for the logging message. :exc_info: An exception tuple, or None. :func: The name of the function or method which invoked the logging call. :sinfo: A stack traceback such as is provided by :func:`traceback.print_stack`, showing the call hierarchy. :kwargs: Additional keyword arguments. .. seealso:: :pep:`282` - A Logging System The proposal which described this feature for inclusion in the Python standard library. `Original Python logging package `_ This is the original source for the :mod:`logging` package. The version of the package available from this site is suitable for use with Python 1.5.2, 2.1.x and 2.2.x, which do not include the :mod:`logging` package in the standard library. .. _logger: Logger Objects -------------- Loggers have the following attributes and methods. Note that Loggers are never instantiated directly, but always through the module-level function ``logging.getLogger(name)``. .. class:: Logger .. attribute:: Logger.propagate If this evaluates to false, logging messages are not passed by this logger or by its child loggers to the handlers of higher level (ancestor) loggers. The constructor sets this attribute to 1. .. method:: Logger.setLevel(lvl) Sets the threshold for this logger to *lvl*. Logging messages which are less severe than *lvl* will be ignored. When a logger is created, the level is set to :const:`NOTSET` (which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger). Note that the root logger is created with level :const:`WARNING`. The term "delegation to the parent" means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached. If an ancestor is found with a level other than NOTSET, then that ancestor's level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled. If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root's level will be used as the effective level. .. method:: Logger.isEnabledFor(lvl) Indicates if a message of severity *lvl* would be processed by this logger. This method checks first the module-level level set by ``logging.disable(lvl)`` and then the logger's effective level as determined by :meth:`getEffectiveLevel`. .. method:: Logger.getEffectiveLevel() Indicates the effective level for this logger. If a value other than :const:`NOTSET` has been set using :meth:`setLevel`, it is returned. Otherwise, the hierarchy is traversed towards the root until a value other than :const:`NOTSET` is found, and that value is returned. .. method:: Logger.getChild(suffix) Returns a logger which is a descendant to this logger, as determined by the suffix. Thus, ``logging.getLogger('abc').getChild('def.ghi')`` would return the same logger as would be returned by ``logging.getLogger('abc.def.ghi')``. This is a convenience method, useful when the parent logger is named using e.g. ``__name__`` rather than a literal string. .. versionadded:: 3.2 .. method:: Logger.debug(msg, *args, **kwargs) Logs a message with level :const:`DEBUG` on this logger. The *msg* is the message format string, and the *args* are the arguments which are merged into *msg* using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.) There are three keyword arguments in *kwargs* which are inspected: *exc_info* which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by :func:`sys.exc_info`) is provided, it is used; otherwise, :func:`sys.exc_info` is called to get the exception information. The second optional keyword argument is *stack_info*, which defaults to False. If specified as True, stack information is added to the logging message, including the actual logging call. Note that this is not the same stack information as that displayed through specifying *exc_info*: The former is stack frames from the bottom of the stack up to the logging call in the current thread, whereas the latter is information about stack frames which have been unwound, following an exception, while searching for exception handlers. You can specify *stack_info* independently of *exc_info*, e.g. to just show how you got to a certain point in your code, even when no exceptions were raised. The stack frames are printed following a header line which says:: Stack (most recent call last): This mimics the `Traceback (most recent call last):` which is used when displaying exception frames. The third keyword argument is *extra* which can be used to pass a dictionary which is used to populate the __dict__ of the LogRecord created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:: FORMAT = "%(asctime)-15s %(clientip)s %(user)-8s %(message)s" logging.basicConfig(format=FORMAT) d = { 'clientip' : '192.168.0.1', 'user' : 'fbloggs' } logger = logging.getLogger("tcpserver") logger.warning("Protocol problem: %s", "connection reset", extra=d) would print something like :: 2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset The keys in the dictionary passed in *extra* should not clash with the keys used by the logging system. (See the :class:`Formatter` documentation for more information on which keys are used by the logging system.) If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the :class:`Formatter` has been set up with a format string which expects 'clientip' and 'user' in the attribute dictionary of the LogRecord. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the *extra* dictionary with these keys. While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized :class:`Formatter`\ s would be used with particular :class:`Handler`\ s. .. versionadded:: 3.2 The *stack_info* parameter was added. .. method:: Logger.info(msg, *args, **kwargs) Logs a message with level :const:`INFO` on this logger. The arguments are interpreted as for :meth:`debug`. .. method:: Logger.warning(msg, *args, **kwargs) Logs a message with level :const:`WARNING` on this logger. The arguments are interpreted as for :meth:`debug`. .. method:: Logger.error(msg, *args, **kwargs) Logs a message with level :const:`ERROR` on this logger. The arguments are interpreted as for :meth:`debug`. .. method:: Logger.critical(msg, *args, **kwargs) Logs a message with level :const:`CRITICAL` on this logger. The arguments are interpreted as for :meth:`debug`. .. method:: Logger.log(lvl, msg, *args, **kwargs) Logs a message with integer level *lvl* on this logger. The other arguments are interpreted as for :meth:`debug`. .. method:: Logger.exception(msg, *args) Logs a message with level :const:`ERROR` on this logger. The arguments are interpreted as for :meth:`debug`. Exception info is added to the logging message. This method should only be called from an exception handler. .. method:: Logger.addFilter(filt) Adds the specified filter *filt* to this logger. .. method:: Logger.removeFilter(filt) Removes the specified filter *filt* from this logger. .. method:: Logger.filter(record) Applies this logger's filters to the record and returns a true value if the record is to be processed. .. method:: Logger.addHandler(hdlr) Adds the specified handler *hdlr* to this logger. .. method:: Logger.removeHandler(hdlr) Removes the specified handler *hdlr* from this logger. .. method:: Logger.findCaller(stack_info=False) Finds the caller's source filename and line number. Returns the filename, line number, function name and stack information as a 4-element tuple. The stack information is returned as *None* unless *stack_info* is *True*. .. method:: Logger.handle(record) Handles a record by passing it to all handlers associated with this logger and its ancestors (until a false value of *propagate* is found). This method is used for unpickled records received from a socket, as well as those created locally. Logger-level filtering is applied using :meth:`~Logger.filter`. .. method:: Logger.makeRecord(name, lvl, fn, lno, msg, args, exc_info, func=None, extra=None, sinfo=None) This is a factory method which can be overridden in subclasses to create specialized :class:`LogRecord` instances. .. method:: Logger.hasHandlers() Checks to see if this logger has any handlers configured. This is done by looking for handlers in this logger and its parents in the logger hierarchy. Returns True if a handler was found, else False. The method stops searching up the hierarchy whenever a logger with the "propagate" attribute set to False is found - that will be the last logger which is checked for the existence of handlers. .. versionadded:: 3.2 .. _minimal-example: Basic example ------------- The :mod:`logging` package provides a lot of flexibility, and its configuration can appear daunting. This section demonstrates that simple use of the logging package is possible. The simplest example shows logging to the console:: import logging logging.debug('A debug message') logging.info('Some information') logging.warning('A shot across the bows') If you run the above script, you'll see this:: WARNING:root:A shot across the bows Because no particular logger was specified, the system used the root logger. The debug and info messages didn't appear because by default, the root logger is configured to only handle messages with a severity of WARNING or above. The message format is also a configuration default, as is the output destination of the messages - ``sys.stderr``. The severity level, the message format and destination can be easily changed, as shown in the example below:: import logging logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)s %(message)s', filename='myapp.log', filemode='w') logging.debug('A debug message') logging.info('Some information') logging.warning('A shot across the bows') The :meth:`basicConfig` method is used to change the configuration defaults, which results in output (written to ``myapp.log``) which should look something like the following:: 2004-07-02 13:00:08,743 DEBUG A debug message 2004-07-02 13:00:08,743 INFO Some information 2004-07-02 13:00:08,743 WARNING A shot across the bows This time, all messages with a severity of DEBUG or above were handled, and the format of the messages was also changed, and output went to the specified file rather than the console. .. XXX logging should probably be updated for new string formatting! Formatting uses the old Python string formatting - see section :ref:`old-string-formatting`. The format string takes the following common specifiers. For a complete list of specifiers, consult the :class:`Formatter` documentation. +-------------------+-----------------------------------------------+ | Format | Description | +===================+===============================================+ | ``%(name)s`` | Name of the logger (logging channel). | +-------------------+-----------------------------------------------+ | ``%(levelname)s`` | Text logging level for the message | | | (``'DEBUG'``, ``'INFO'``, ``'WARNING'``, | | | ``'ERROR'``, ``'CRITICAL'``). | +-------------------+-----------------------------------------------+ | ``%(asctime)s`` | Human-readable time when the | | | :class:`LogRecord` was created. By default | | | this is of the form "2003-07-08 16:49:45,896" | | | (the numbers after the comma are millisecond | | | portion of the time). | +-------------------+-----------------------------------------------+ | ``%(message)s`` | The logged message. | +-------------------+-----------------------------------------------+ To change the date/time format, you can pass an additional keyword parameter, *datefmt*, as in the following:: import logging logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)-8s %(message)s', datefmt='%a, %d %b %Y %H:%M:%S', filename='/temp/myapp.log', filemode='w') logging.debug('A debug message') logging.info('Some information') logging.warning('A shot across the bows') which would result in output like :: Fri, 02 Jul 2004 13:06:18 DEBUG A debug message Fri, 02 Jul 2004 13:06:18 INFO Some information Fri, 02 Jul 2004 13:06:18 WARNING A shot across the bows The date format string follows the requirements of :func:`strftime` - see the documentation for the :mod:`time` module. If, instead of sending logging output to the console or a file, you'd rather use a file-like object which you have created separately, you can pass it to :func:`basicConfig` using the *stream* keyword argument. Note that if both *stream* and *filename* keyword arguments are passed, the *stream* argument is ignored. Of course, you can put variable information in your output. To do this, simply have the message be a format string and pass in additional arguments containing the variable information, as in the following example:: import logging logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)-8s %(message)s', datefmt='%a, %d %b %Y %H:%M:%S', filename='/temp/myapp.log', filemode='w') logging.error('Pack my box with %d dozen %s', 5, 'liquor jugs') which would result in :: Wed, 21 Jul 2004 15:35:16 ERROR Pack my box with 5 dozen liquor jugs .. _multiple-destinations: Logging to multiple destinations -------------------------------- Let's say you want to log to console and file with different message formats and in differing circumstances. Say you want to log messages with levels of DEBUG and higher to file, and those messages at level INFO and higher to the console. Let's also assume that the file should contain timestamps, but the console messages should not. Here's how you can achieve this:: import logging # set up logging to file - see previous section for more details logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(name)-12s %(levelname)-8s %(message)s', datefmt='%m-%d %H:%M', filename='/temp/myapp.log', filemode='w') # define a Handler which writes INFO messages or higher to the sys.stderr console = logging.StreamHandler() console.setLevel(logging.INFO) # set a format which is simpler for console use formatter = logging.Formatter('%(name)-12s: %(levelname)-8s %(message)s') # tell the handler to use this format console.setFormatter(formatter) # add the handler to the root logger logging.getLogger('').addHandler(console) # Now, we can log to the root logger, or any other logger. First the root... logging.info('Jackdaws love my big sphinx of quartz.') # Now, define a couple of other loggers which might represent areas in your # application: logger1 = logging.getLogger('myapp.area1') logger2 = logging.getLogger('myapp.area2') logger1.debug('Quick zephyrs blow, vexing daft Jim.') logger1.info('How quickly daft jumping zebras vex.') logger2.warning('Jail zesty vixen who grabbed pay from quack.') logger2.error('The five boxing wizards jump quickly.') When you run this, on the console you will see :: root : INFO Jackdaws love my big sphinx of quartz. myapp.area1 : INFO How quickly daft jumping zebras vex. myapp.area2 : WARNING Jail zesty vixen who grabbed pay from quack. myapp.area2 : ERROR The five boxing wizards jump quickly. and in the file you will see something like :: 10-22 22:19 root INFO Jackdaws love my big sphinx of quartz. 10-22 22:19 myapp.area1 DEBUG Quick zephyrs blow, vexing daft Jim. 10-22 22:19 myapp.area1 INFO How quickly daft jumping zebras vex. 10-22 22:19 myapp.area2 WARNING Jail zesty vixen who grabbed pay from quack. 10-22 22:19 myapp.area2 ERROR The five boxing wizards jump quickly. As you can see, the DEBUG message only shows up in the file. The other messages are sent to both destinations. This example uses console and file handlers, but you can use any number and combination of handlers you choose. .. _logging-exceptions: Exceptions raised during logging -------------------------------- The logging package is designed to swallow exceptions which occur while logging in production. This is so that errors which occur while handling logging events - such as logging misconfiguration, network or other similar errors - do not cause the application using logging to terminate prematurely. :class:`SystemExit` and :class:`KeyboardInterrupt` exceptions are never swallowed. Other exceptions which occur during the :meth:`emit` method of a :class:`Handler` subclass are passed to its :meth:`handleError` method. The default implementation of :meth:`handleError` in :class:`Handler` checks to see if a module-level variable, :data:`raiseExceptions`, is set. If set, a traceback is printed to :data:`sys.stderr`. If not set, the exception is swallowed. **Note:** The default value of :data:`raiseExceptions` is ``True``. This is because during development, you typically want to be notified of any exceptions that occur. It's advised that you set :data:`raiseExceptions` to ``False`` for production usage. .. _context-info: Adding contextual information to your logging output ---------------------------------------------------- Sometimes you want logging output to contain contextual information in addition to the parameters passed to the logging call. For example, in a networked application, it may be desirable to log client-specific information in the log (e.g. remote client's username, or IP address). Although you could use the *extra* parameter to achieve this, it's not always convenient to pass the information in this way. While it might be tempting to create :class:`Logger` instances on a per-connection basis, this is not a good idea because these instances are not garbage collected. While this is not a problem in practice, when the number of :class:`Logger` instances is dependent on the level of granularity you want to use in logging an application, it could be hard to manage if the number of :class:`Logger` instances becomes effectively unbounded. Using LoggerAdapters to impart contextual information ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ An easy way in which you can pass contextual information to be output along with logging event information is to use the :class:`LoggerAdapter` class. This class is designed to look like a :class:`Logger`, so that you can call :meth:`debug`, :meth:`info`, :meth:`warning`, :meth:`error`, :meth:`exception`, :meth:`critical` and :meth:`log`. These methods have the same signatures as their counterparts in :class:`Logger`, so you can use the two types of instances interchangeably. When you create an instance of :class:`LoggerAdapter`, you pass it a :class:`Logger` instance and a dict-like object which contains your contextual information. When you call one of the logging methods on an instance of :class:`LoggerAdapter`, it delegates the call to the underlying instance of :class:`Logger` passed to its constructor, and arranges to pass the contextual information in the delegated call. Here's a snippet from the code of :class:`LoggerAdapter`:: def debug(self, msg, *args, **kwargs): """ Delegate a debug call to the underlying logger, after adding contextual information from this adapter instance. """ msg, kwargs = self.process(msg, kwargs) self.logger.debug(msg, *args, **kwargs) The :meth:`process` method of :class:`LoggerAdapter` is where the contextual information is added to the logging output. It's passed the message and keyword arguments of the logging call, and it passes back (potentially) modified versions of these to use in the call to the underlying logger. The default implementation of this method leaves the message alone, but inserts an "extra" key in the keyword argument whose value is the dict-like object passed to the constructor. Of course, if you had passed an "extra" keyword argument in the call to the adapter, it will be silently overwritten. The advantage of using "extra" is that the values in the dict-like object are merged into the :class:`LogRecord` instance's __dict__, allowing you to use customized strings with your :class:`Formatter` instances which know about the keys of the dict-like object. If you need a different method, e.g. if you want to prepend or append the contextual information to the message string, you just need to subclass :class:`LoggerAdapter` and override :meth:`process` to do what you need. Here's an example script which uses this class, which also illustrates what dict-like behaviour is needed from an arbitrary "dict-like" object for use in the constructor:: import logging class ConnInfo: """ An example class which shows how an arbitrary class can be used as the 'extra' context information repository passed to a LoggerAdapter. """ def __getitem__(self, name): """ To allow this instance to look like a dict. """ from random import choice if name == "ip": result = choice(["127.0.0.1", "192.168.0.1"]) elif name == "user": result = choice(["jim", "fred", "sheila"]) else: result = self.__dict__.get(name, "?") return result def __iter__(self): """ To allow iteration over keys, which will be merged into the LogRecord dict before formatting and output. """ keys = ["ip", "user"] keys.extend(self.__dict__.keys()) return keys.__iter__() if __name__ == "__main__": from random import choice levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL) a1 = logging.LoggerAdapter(logging.getLogger("a.b.c"), { "ip" : "123.231.231.123", "user" : "sheila" }) logging.basicConfig(level=logging.DEBUG, format="%(asctime)-15s %(name)-5s %(levelname)-8s IP: %(ip)-15s User: %(user)-8s %(message)s") a1.debug("A debug message") a1.info("An info message with %s", "some parameters") a2 = logging.LoggerAdapter(logging.getLogger("d.e.f"), ConnInfo()) for x in range(10): lvl = choice(levels) lvlname = logging.getLevelName(lvl) a2.log(lvl, "A message at %s level with %d %s", lvlname, 2, "parameters") When this script is run, the output should look something like this:: 2008-01-18 14:49:54,023 a.b.c DEBUG IP: 123.231.231.123 User: sheila A debug message 2008-01-18 14:49:54,023 a.b.c INFO IP: 123.231.231.123 User: sheila An info message with some parameters 2008-01-18 14:49:54,023 d.e.f CRITICAL IP: 192.168.0.1 User: jim A message at CRITICAL level with 2 parameters 2008-01-18 14:49:54,033 d.e.f INFO IP: 192.168.0.1 User: jim A message at INFO level with 2 parameters 2008-01-18 14:49:54,033 d.e.f WARNING IP: 192.168.0.1 User: sheila A message at WARNING level with 2 parameters 2008-01-18 14:49:54,033 d.e.f ERROR IP: 127.0.0.1 User: fred A message at ERROR level with 2 parameters 2008-01-18 14:49:54,033 d.e.f ERROR IP: 127.0.0.1 User: sheila A message at ERROR level with 2 parameters 2008-01-18 14:49:54,033 d.e.f WARNING IP: 192.168.0.1 User: sheila A message at WARNING level with 2 parameters 2008-01-18 14:49:54,033 d.e.f WARNING IP: 192.168.0.1 User: jim A message at WARNING level with 2 parameters 2008-01-18 14:49:54,033 d.e.f INFO IP: 192.168.0.1 User: fred A message at INFO level with 2 parameters 2008-01-18 14:49:54,033 d.e.f WARNING IP: 192.168.0.1 User: sheila A message at WARNING level with 2 parameters 2008-01-18 14:49:54,033 d.e.f WARNING IP: 127.0.0.1 User: jim A message at WARNING level with 2 parameters .. _filters-contextual: Using Filters to impart contextual information ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You can also add contextual information to log output using a user-defined :class:`Filter`. ``Filter`` instances are allowed to modify the ``LogRecords`` passed to them, including adding additional attributes which can then be output using a suitable format string, or if needed a custom :class:`Formatter`. For example in a web application, the request being processed (or at least, the interesting parts of it) can be stored in a threadlocal (:class:`threading.local`) variable, and then accessed from a ``Filter`` to add, say, information from the request - say, the remote IP address and remote user's username - to the ``LogRecord``, using the attribute names 'ip' and 'user' as in the ``LoggerAdapter`` example above. In that case, the same format string can be used to get similar output to that shown above. Here's an example script:: import logging from random import choice class ContextFilter(logging.Filter): """ This is a filter which injects contextual information into the log. Rather than use actual contextual information, we just use random data in this demo. """ USERS = ['jim', 'fred', 'sheila'] IPS = ['123.231.231.123', '127.0.0.1', '192.168.0.1'] def filter(self, record): record.ip = choice(ContextFilter.IPS) record.user = choice(ContextFilter.USERS) return True if __name__ == "__main__": levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL) a1 = logging.LoggerAdapter(logging.getLogger("a.b.c"), { "ip" : "123.231.231.123", "user" : "sheila" }) logging.basicConfig(level=logging.DEBUG, format="%(asctime)-15s %(name)-5s %(levelname)-8s IP: %(ip)-15s User: %(user)-8s %(message)s") a1 = logging.getLogger("a.b.c") a2 = logging.getLogger("d.e.f") f = ContextFilter() a1.addFilter(f) a2.addFilter(f) a1.debug("A debug message") a1.info("An info message with %s", "some parameters") for x in range(10): lvl = choice(levels) lvlname = logging.getLevelName(lvl) a2.log(lvl, "A message at %s level with %d %s", lvlname, 2, "parameters") which, when run, produces something like:: 2010-09-06 22:38:15,292 a.b.c DEBUG IP: 123.231.231.123 User: fred A debug message 2010-09-06 22:38:15,300 a.b.c INFO IP: 192.168.0.1 User: sheila An info message with some parameters 2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1 User: sheila A message at CRITICAL level with 2 parameters 2010-09-06 22:38:15,300 d.e.f ERROR IP: 127.0.0.1 User: jim A message at ERROR level with 2 parameters 2010-09-06 22:38:15,300 d.e.f DEBUG IP: 127.0.0.1 User: sheila A message at DEBUG level with 2 parameters 2010-09-06 22:38:15,300 d.e.f ERROR IP: 123.231.231.123 User: fred A message at ERROR level with 2 parameters 2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 192.168.0.1 User: jim A message at CRITICAL level with 2 parameters 2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1 User: sheila A message at CRITICAL level with 2 parameters 2010-09-06 22:38:15,300 d.e.f DEBUG IP: 192.168.0.1 User: jim A message at DEBUG level with 2 parameters 2010-09-06 22:38:15,301 d.e.f ERROR IP: 127.0.0.1 User: sheila A message at ERROR level with 2 parameters 2010-09-06 22:38:15,301 d.e.f DEBUG IP: 123.231.231.123 User: fred A message at DEBUG level with 2 parameters 2010-09-06 22:38:15,301 d.e.f INFO IP: 123.231.231.123 User: fred A message at INFO level with 2 parameters .. _multiple-processes: Logging to a single file from multiple processes ------------------------------------------------ Although logging is thread-safe, and logging to a single file from multiple threads in a single process *is* supported, logging to a single file from *multiple processes* is *not* supported, because there is no standard way to serialize access to a single file across multiple processes in Python. If you need to log to a single file from multiple processes, one way of doing this is to have all the processes log to a :class:`SocketHandler`, and have a separate process which implements a socket server which reads from the socket and logs to file. (If you prefer, you can dedicate one thread in one of the existing processes to perform this function.) The following section documents this approach in more detail and includes a working socket receiver which can be used as a starting point for you to adapt in your own applications. If you are using a recent version of Python which includes the :mod:`multiprocessing` module, you could write your own handler which uses the :class:`Lock` class from this module to serialize access to the file from your processes. The existing :class:`FileHandler` and subclasses do not make use of :mod:`multiprocessing` at present, though they may do so in the future. Note that at present, the :mod:`multiprocessing` module does not provide working lock functionality on all platforms (see http://bugs.python.org/issue3770). .. currentmodule:: logging.handlers Alternatively, you can use a ``Queue`` and a :class:`QueueHandler` to send all logging events to one of the processes in your multi-process application. The following example script demonstrates how you can do this; in the example a separate listener process listens for events sent by other processes and logs them according to its own logging configuration. Although the example only demonstrates one way of doing it (for example, you may want to use a listener thread rather than a separate listener process - the implementation would be analogous) it does allow for completely different logging configurations for the listener and the other processes in your application, and can be used as the basis for code meeting your own specific requirements:: # You'll need these imports in your own code import logging import logging.handlers import multiprocessing # Next two import lines for this demo only from random import choice, random import time # # Because you'll want to define the logging configurations for listener and workers, the # listener and worker process functions take a configurer parameter which is a callable # for configuring logging for that process. These functions are also passed the queue, # which they use for communication. # # In practice, you can configure the listener however you want, but note that in this # simple example, the listener does not apply level or filter logic to received records. # In practice, you would probably want to do ths logic in the worker processes, to avoid # sending events which would be filtered out between processes. # # The size of the rotated files is made small so you can see the results easily. def listener_configurer(): root = logging.getLogger() h = logging.handlers.RotatingFileHandler('/tmp/mptest.log', 'a', 300, 10) f = logging.Formatter('%(asctime)s %(processName)-10s %(name)s %(levelname)-8s %(message)s') h.setFormatter(f) root.addHandler(h) # This is the listener process top-level loop: wait for logging events # (LogRecords)on the queue and handle them, quit when you get a None for a # LogRecord. def listener_process(queue, configurer): configurer() while True: try: record = queue.get() if record is None: # We send this as a sentinel to tell the listener to quit. break logger = logging.getLogger(record.name) logger.handle(record) # No level or filter logic applied - just do it! except (KeyboardInterrupt, SystemExit): raise except: import sys, traceback print >> sys.stderr, 'Whoops! Problem:' traceback.print_exc(file=sys.stderr) # Arrays used for random selections in this demo LEVELS = [logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL] LOGGERS = ['a.b.c', 'd.e.f'] MESSAGES = [ 'Random message #1', 'Random message #2', 'Random message #3', ] # The worker configuration is done at the start of the worker process run. # Note that on Windows you can't rely on fork semantics, so each process # will run the logging configuration code when it starts. def worker_configurer(queue): h = logging.handlers.QueueHandler(queue) # Just the one handler needed root = logging.getLogger() root.addHandler(h) root.setLevel(logging.DEBUG) # send all messages, for demo; no other level or filter logic applied. # This is the worker process top-level loop, which just logs ten events with # random intervening delays before terminating. # The print messages are just so you know it's doing something! def worker_process(queue, configurer): configurer(queue) name = multiprocessing.current_process().name print('Worker started: %s' % name) for i in range(10): time.sleep(random()) logger = logging.getLogger(choice(LOGGERS)) level = choice(LEVELS) message = choice(MESSAGES) logger.log(level, message) print('Worker finished: %s' % name) # Here's where the demo gets orchestrated. Create the queue, create and start # the listener, create ten workers and start them, wait for them to finish, # then send a None to the queue to tell the listener to finish. def main(): queue = multiprocessing.Queue(-1) listener = multiprocessing.Process(target=listener_process, args=(queue, listener_configurer)) listener.start() workers = [] for i in range(10): worker = multiprocessing.Process(target=worker_process, args=(queue, worker_configurer)) workers.append(worker) worker.start() for w in workers: w.join() queue.put_nowait(None) listener.join() if __name__ == '__main__': main() .. currentmodule:: logging .. _network-logging: Sending and receiving logging events across a network ----------------------------------------------------- Let's say you want to send logging events across a network, and handle them at the receiving end. A simple way of doing this is attaching a :class:`SocketHandler` instance to the root logger at the sending end:: import logging, logging.handlers rootLogger = logging.getLogger('') rootLogger.setLevel(logging.DEBUG) socketHandler = logging.handlers.SocketHandler('localhost', logging.handlers.DEFAULT_TCP_LOGGING_PORT) # don't bother with a formatter, since a socket handler sends the event as # an unformatted pickle rootLogger.addHandler(socketHandler) # Now, we can log to the root logger, or any other logger. First the root... logging.info('Jackdaws love my big sphinx of quartz.') # Now, define a couple of other loggers which might represent areas in your # application: logger1 = logging.getLogger('myapp.area1') logger2 = logging.getLogger('myapp.area2') logger1.debug('Quick zephyrs blow, vexing daft Jim.') logger1.info('How quickly daft jumping zebras vex.') logger2.warning('Jail zesty vixen who grabbed pay from quack.') logger2.error('The five boxing wizards jump quickly.') At the receiving end, you can set up a receiver using the :mod:`socketserver` module. Here is a basic working example:: import pickle import logging import logging.handlers import socketserver import struct class LogRecordStreamHandler(socketserver.StreamRequestHandler): """Handler for a streaming logging request. This basically logs the record using whatever logging policy is configured locally. """ def handle(self): """ Handle multiple requests - each expected to be a 4-byte length, followed by the LogRecord in pickle format. Logs the record according to whatever policy is configured locally. """ while True: chunk = self.connection.recv(4) if len(chunk) < 4: break slen = struct.unpack(">L", chunk)[0] chunk = self.connection.recv(slen) while len(chunk) < slen: chunk = chunk + self.connection.recv(slen - len(chunk)) obj = self.unPickle(chunk) record = logging.makeLogRecord(obj) self.handleLogRecord(record) def unPickle(self, data): return pickle.loads(data) def handleLogRecord(self, record): # if a name is specified, we use the named logger rather than the one # implied by the record. if self.server.logname is not None: name = self.server.logname else: name = record.name logger = logging.getLogger(name) # N.B. EVERY record gets logged. This is because Logger.handle # is normally called AFTER logger-level filtering. If you want # to do filtering, do it at the client end to save wasting # cycles and network bandwidth! logger.handle(record) class LogRecordSocketReceiver(socketserver.ThreadingTCPServer): """simple TCP socket-based logging receiver suitable for testing. """ allow_reuse_address = 1 def __init__(self, host='localhost', port=logging.handlers.DEFAULT_TCP_LOGGING_PORT, handler=LogRecordStreamHandler): socketserver.ThreadingTCPServer.__init__(self, (host, port), handler) self.abort = 0 self.timeout = 1 self.logname = None def serve_until_stopped(self): import select abort = 0 while not abort: rd, wr, ex = select.select([self.socket.fileno()], [], [], self.timeout) if rd: self.handle_request() abort = self.abort def main(): logging.basicConfig( format="%(relativeCreated)5d %(name)-15s %(levelname)-8s %(message)s") tcpserver = LogRecordSocketReceiver() print("About to start TCP server...") tcpserver.serve_until_stopped() if __name__ == "__main__": main() First run the server, and then the client. On the client side, nothing is printed on the console; on the server side, you should see something like:: About to start TCP server... 59 root INFO Jackdaws love my big sphinx of quartz. 59 myapp.area1 DEBUG Quick zephyrs blow, vexing daft Jim. 69 myapp.area1 INFO How quickly daft jumping zebras vex. 69 myapp.area2 WARNING Jail zesty vixen who grabbed pay from quack. 69 myapp.area2 ERROR The five boxing wizards jump quickly. Note that there are some security issues with pickle in some scenarios. If these affect you, you can use an alternative serialization scheme by overriding the :meth:`makePickle` method and implementing your alternative there, as well as adapting the above script to use your alternative serialization. .. _arbitrary-object-messages: Using arbitrary objects as messages ----------------------------------- In the preceding sections and examples, it has been assumed that the message passed when logging the event is a string. However, this is not the only possibility. You can pass an arbitrary object as a message, and its :meth:`__str__` method will be called when the logging system needs to convert it to a string representation. In fact, if you want to, you can avoid computing a string representation altogether - for example, the :class:`SocketHandler` emits an event by pickling it and sending it over the wire. Dealing with handlers that block -------------------------------- .. currentmodule:: logging.handlers Sometimes you have to get your logging handlers to do their work without blocking the thread you’re logging from. This is common in Web applications, though of course it also occurs in other scenarios. A common culprit which demonstrates sluggish behaviour is the :class:`SMTPHandler`: sending emails can take a long time, for a number of reasons outside the developer’s control (for example, a poorly performing mail or network infrastructure). But almost any network-based handler can block: Even a :class:`SocketHandler` operation may do a DNS query under the hood which is too slow (and this query can be deep in the socket library code, below the Python layer, and outside your control). One solution is to use a two-part approach. For the first part, attach only a :class:`QueueHandler` to those loggers which are accessed from performance-critical threads. They simply write to their queue, which can be sized to a large enough capacity or initialized with no upper bound to their size. The write to the queue will typically be accepted quickly, though you will probably need to catch the :ref:`queue.Full` exception as a precaution in your code. If you are a library developer who has performance-critical threads in their code, be sure to document this (together with a suggestion to attach only ``QueueHandlers`` to your loggers) for the benefit of other developers who will use your code. The second part of the solution is :class:`QueueListener`, which has been designed as the counterpart to :class:`QueueHandler`. A :class:`QueueListener` is very simple: it’s passed a queue and some handlers, and it fires up an internal thread which listens to its queue for LogRecords sent from ``QueueHandlers`` (or any other source of ``LogRecords``, for that matter). The ``LogRecords`` are removed from the queue and passed to the handlers for processing. The advantage of having a separate :class:`QueueListener` class is that you can use the same instance to service multiple ``QueueHandlers``. This is more resource-friendly than, say, having threaded versions of the existing handler classes, which would eat up one thread per handler for no particular benefit. An example of using these two classes follows (imports omitted):: que = queue.Queue(-1) # no limit on size queue_handler = QueueHandler(que) handler = logging.StreamHandler() listener = QueueListener(que, handler) root = logging.getLogger() root.addHandler(queue_handler) formatter = logging.Formatter('%(threadName)s: %(message)s') handler.setFormatter(formatter) listener.start() # The log output will display the thread which generated # the event (the main thread) rather than the internal # thread which monitors the internal queue. This is what # you want to happen. root.warning('Look out!') listener.stop() which, when run, will produce:: MainThread: Look out! Optimization ------------ Formatting of message arguments is deferred until it cannot be avoided. However, computing the arguments passed to the logging method can also be expensive, and you may want to avoid doing it if the logger will just throw away your event. To decide what to do, you can call the :meth:`isEnabledFor` method which takes a level argument and returns true if the event would be created by the Logger for that level of call. You can write code like this:: if logger.isEnabledFor(logging.DEBUG): logger.debug("Message with %s, %s", expensive_func1(), expensive_func2()) so that if the logger's threshold is set above ``DEBUG``, the calls to :func:`expensive_func1` and :func:`expensive_func2` are never made. There are other optimizations which can be made for specific applications which need more precise control over what logging information is collected. Here's a list of things you can do to avoid processing during logging which you don't need: +-----------------------------------------------+----------------------------------------+ | What you don't want to collect | How to avoid collecting it | +===============================================+========================================+ | Information about where calls were made from. | Set ``logging._srcfile`` to ``None``. | +-----------------------------------------------+----------------------------------------+ | Threading information. | Set ``logging.logThreads`` to ``0``. | +-----------------------------------------------+----------------------------------------+ | Process information. | Set ``logging.logProcesses`` to ``0``. | +-----------------------------------------------+----------------------------------------+ Also note that the core logging module only includes the basic handlers. If you don't import :mod:`logging.handlers` and :mod:`logging.config`, they won't take up any memory. .. _handler: Handler Objects --------------- Handlers have the following attributes and methods. Note that :class:`Handler` is never instantiated directly; this class acts as a base for more useful subclasses. However, the :meth:`__init__` method in subclasses needs to call :meth:`Handler.__init__`. .. method:: Handler.__init__(level=NOTSET) Initializes the :class:`Handler` instance by setting its level, setting the list of filters to the empty list and creating a lock (using :meth:`createLock`) for serializing access to an I/O mechanism. .. method:: Handler.createLock() Initializes a thread lock which can be used to serialize access to underlying I/O functionality which may not be threadsafe. .. method:: Handler.acquire() Acquires the thread lock created with :meth:`createLock`. .. method:: Handler.release() Releases the thread lock acquired with :meth:`acquire`. .. method:: Handler.setLevel(lvl) Sets the threshold for this handler to *lvl*. Logging messages which are less severe than *lvl* will be ignored. When a handler is created, the level is set to :const:`NOTSET` (which causes all messages to be processed). .. method:: Handler.setFormatter(form) Sets the :class:`Formatter` for this handler to *form*. .. method:: Handler.addFilter(filt) Adds the specified filter *filt* to this handler. .. method:: Handler.removeFilter(filt) Removes the specified filter *filt* from this handler. .. method:: Handler.filter(record) Applies this handler's filters to the record and returns a true value if the record is to be processed. .. method:: Handler.flush() Ensure all logging output has been flushed. This version does nothing and is intended to be implemented by subclasses. .. method:: Handler.close() Tidy up any resources used by the handler. This version does no output but removes the handler from an internal list of handlers which is closed when :func:`shutdown` is called. Subclasses should ensure that this gets called from overridden :meth:`close` methods. .. method:: Handler.handle(record) Conditionally emits the specified logging record, depending on filters which may have been added to the handler. Wraps the actual emission of the record with acquisition/release of the I/O thread lock. .. method:: Handler.handleError(record) This method should be called from handlers when an exception is encountered during an :meth:`emit` call. By default it does nothing, which means that exceptions get silently ignored. This is what is mostly wanted for a logging system - most users will not care about errors in the logging system, they are more interested in application errors. You could, however, replace this with a custom handler if you wish. The specified record is the one which was being processed when the exception occurred. .. method:: Handler.format(record) Do formatting for a record - if a formatter is set, use it. Otherwise, use the default formatter for the module. .. method:: Handler.emit(record) Do whatever it takes to actually log the specified logging record. This version is intended to be implemented by subclasses and so raises a :exc:`NotImplementedError`. .. _stream-handler: StreamHandler ^^^^^^^^^^^^^ The :class:`StreamHandler` class, located in the core :mod:`logging` package, sends logging output to streams such as *sys.stdout*, *sys.stderr* or any file-like object (or, more precisely, any object which supports :meth:`write` and :meth:`flush` methods). .. currentmodule:: logging .. class:: StreamHandler(stream=None) Returns a new instance of the :class:`StreamHandler` class. If *stream* is specified, the instance will use it for logging output; otherwise, *sys.stderr* will be used. .. method:: emit(record) If a formatter is specified, it is used to format the record. The record is then written to the stream with a trailing newline. If exception information is present, it is formatted using :func:`traceback.print_exception` and appended to the stream. .. method:: flush() Flushes the stream by calling its :meth:`flush` method. Note that the :meth:`close` method is inherited from :class:`Handler` and so does no output, so an explicit :meth:`flush` call may be needed at times. .. versionchanged:: 3.2 The ``StreamHandler`` class now has a ``terminator`` attribute, default value ``"\n"``, which is used as the terminator when writing a formatted record to a stream. If you don't want this newline termination, you can set the handler instance's ``terminator`` attribute to the empty string. .. _file-handler: FileHandler ^^^^^^^^^^^ The :class:`FileHandler` class, located in the core :mod:`logging` package, sends logging output to a disk file. It inherits the output functionality from :class:`StreamHandler`. .. class:: FileHandler(filename, mode='a', encoding=None, delay=False) Returns a new instance of the :class:`FileHandler` class. The specified file is opened and used as the stream for logging. If *mode* is not specified, :const:`'a'` is used. If *encoding* is not *None*, it is used to open the file with that encoding. If *delay* is true, then file opening is deferred until the first call to :meth:`emit`. By default, the file grows indefinitely. .. method:: close() Closes the file. .. method:: emit(record) Outputs the record to the file. .. _null-handler: NullHandler ^^^^^^^^^^^ .. versionadded:: 3.1 The :class:`NullHandler` class, located in the core :mod:`logging` package, does not do any formatting or output. It is essentially a "no-op" handler for use by library developers. .. class:: NullHandler() Returns a new instance of the :class:`NullHandler` class. .. method:: emit(record) This method does nothing. .. method:: handle(record) This method does nothing. .. method:: createLock() This method returns ``None`` for the lock, since there is no underlying I/O to which access needs to be serialized. See :ref:`library-config` for more information on how to use :class:`NullHandler`. .. _watched-file-handler: WatchedFileHandler ^^^^^^^^^^^^^^^^^^ .. currentmodule:: logging.handlers The :class:`WatchedFileHandler` class, located in the :mod:`logging.handlers` module, is a :class:`FileHandler` which watches the file it is logging to. If the file changes, it is closed and reopened using the file name. A file change can happen because of usage of programs such as *newsyslog* and *logrotate* which perform log file rotation. This handler, intended for use under Unix/Linux, watches the file to see if it has changed since the last emit. (A file is deemed to have changed if its device or inode have changed.) If the file has changed, the old file stream is closed, and the file opened to get a new stream. This handler is not appropriate for use under Windows, because under Windows open log files cannot be moved or renamed - logging opens the files with exclusive locks - and so there is no need for such a handler. Furthermore, *ST_INO* is not supported under Windows; :func:`stat` always returns zero for this value. .. class:: WatchedFileHandler(filename[,mode[, encoding[, delay]]]) Returns a new instance of the :class:`WatchedFileHandler` class. The specified file is opened and used as the stream for logging. If *mode* is not specified, :const:`'a'` is used. If *encoding* is not *None*, it is used to open the file with that encoding. If *delay* is true, then file opening is deferred until the first call to :meth:`emit`. By default, the file grows indefinitely. .. method:: emit(record) Outputs the record to the file, but first checks to see if the file has changed. If it has, the existing stream is flushed and closed and the file opened again, before outputting the record to the file. .. _rotating-file-handler: RotatingFileHandler ^^^^^^^^^^^^^^^^^^^ The :class:`RotatingFileHandler` class, located in the :mod:`logging.handlers` module, supports rotation of disk log files. .. class:: RotatingFileHandler(filename, mode='a', maxBytes=0, backupCount=0, encoding=None, delay=0) Returns a new instance of the :class:`RotatingFileHandler` class. The specified file is opened and used as the stream for logging. If *mode* is not specified, ``'a'`` is used. If *encoding* is not *None*, it is used to open the file with that encoding. If *delay* is true, then file opening is deferred until the first call to :meth:`emit`. By default, the file grows indefinitely. You can use the *maxBytes* and *backupCount* values to allow the file to :dfn:`rollover` at a predetermined size. When the size is about to be exceeded, the file is closed and a new file is silently opened for output. Rollover occurs whenever the current log file is nearly *maxBytes* in length; if *maxBytes* is zero, rollover never occurs. If *backupCount* is non-zero, the system will save old log files by appending the extensions ".1", ".2" etc., to the filename. For example, with a *backupCount* of 5 and a base file name of :file:`app.log`, you would get :file:`app.log`, :file:`app.log.1`, :file:`app.log.2`, up to :file:`app.log.5`. The file being written to is always :file:`app.log`. When this file is filled, it is closed and renamed to :file:`app.log.1`, and if files :file:`app.log.1`, :file:`app.log.2`, etc. exist, then they are renamed to :file:`app.log.2`, :file:`app.log.3` etc. respectively. .. method:: doRollover() Does a rollover, as described above. .. method:: emit(record) Outputs the record to the file, catering for rollover as described previously. .. _timed-rotating-file-handler: TimedRotatingFileHandler ^^^^^^^^^^^^^^^^^^^^^^^^ The :class:`TimedRotatingFileHandler` class, located in the :mod:`logging.handlers` module, supports rotation of disk log files at certain timed intervals. .. class:: TimedRotatingFileHandler(filename, when='h', interval=1, backupCount=0, encoding=None, delay=False, utc=False) Returns a new instance of the :class:`TimedRotatingFileHandler` class. The specified file is opened and used as the stream for logging. On rotating it also sets the filename suffix. Rotating happens based on the product of *when* and *interval*. You can use the *when* to specify the type of *interval*. The list of possible values is below. Note that they are not case sensitive. +----------------+-----------------------+ | Value | Type of interval | +================+=======================+ | ``'S'`` | Seconds | +----------------+-----------------------+ | ``'M'`` | Minutes | +----------------+-----------------------+ | ``'H'`` | Hours | +----------------+-----------------------+ | ``'D'`` | Days | +----------------+-----------------------+ | ``'W'`` | Week day (0=Monday) | +----------------+-----------------------+ | ``'midnight'`` | Roll over at midnight | +----------------+-----------------------+ The system will save old log files by appending extensions to the filename. The extensions are date-and-time based, using the strftime format ``%Y-%m-%d_%H-%M-%S`` or a leading portion thereof, depending on the rollover interval. When computing the next rollover time for the first time (when the handler is created), the last modification time of an existing log file, or else the current time, is used to compute when the next rotation will occur. If the *utc* argument is true, times in UTC will be used; otherwise local time is used. If *backupCount* is nonzero, at most *backupCount* files will be kept, and if more would be created when rollover occurs, the oldest one is deleted. The deletion logic uses the interval to determine which files to delete, so changing the interval may leave old files lying around. If *delay* is true, then file opening is deferred until the first call to :meth:`emit`. .. method:: doRollover() Does a rollover, as described above. .. method:: emit(record) Outputs the record to the file, catering for rollover as described above. .. _socket-handler: SocketHandler ^^^^^^^^^^^^^ The :class:`SocketHandler` class, located in the :mod:`logging.handlers` module, sends logging output to a network socket. The base class uses a TCP socket. .. class:: SocketHandler(host, port) Returns a new instance of the :class:`SocketHandler` class intended to communicate with a remote machine whose address is given by *host* and *port*. .. method:: close() Closes the socket. .. method:: emit() Pickles the record's attribute dictionary and writes it to the socket in binary format. If there is an error with the socket, silently drops the packet. If the connection was previously lost, re-establishes the connection. To unpickle the record at the receiving end into a :class:`LogRecord`, use the :func:`makeLogRecord` function. .. method:: handleError() Handles an error which has occurred during :meth:`emit`. The most likely cause is a lost connection. Closes the socket so that we can retry on the next event. .. method:: makeSocket() This is a factory method which allows subclasses to define the precise type of socket they want. The default implementation creates a TCP socket (:const:`socket.SOCK_STREAM`). .. method:: makePickle(record) Pickles the record's attribute dictionary in binary format with a length prefix, and returns it ready for transmission across the socket. Note that pickles aren't completely secure. If you are concerned about security, you may want to override this method to implement a more secure mechanism. For example, you can sign pickles using HMAC and then verify them on the receiving end, or alternatively you can disable unpickling of global objects on the receiving end. .. method:: send(packet) Send a pickled string *packet* to the socket. This function allows for partial sends which can happen when the network is busy. .. _datagram-handler: DatagramHandler ^^^^^^^^^^^^^^^ The :class:`DatagramHandler` class, located in the :mod:`logging.handlers` module, inherits from :class:`SocketHandler` to support sending logging messages over UDP sockets. .. class:: DatagramHandler(host, port) Returns a new instance of the :class:`DatagramHandler` class intended to communicate with a remote machine whose address is given by *host* and *port*. .. method:: emit() Pickles the record's attribute dictionary and writes it to the socket in binary format. If there is an error with the socket, silently drops the packet. To unpickle the record at the receiving end into a :class:`LogRecord`, use the :func:`makeLogRecord` function. .. method:: makeSocket() The factory method of :class:`SocketHandler` is here overridden to create a UDP socket (:const:`socket.SOCK_DGRAM`). .. method:: send(s) Send a pickled string to a socket. .. _syslog-handler: SysLogHandler ^^^^^^^^^^^^^ The :class:`SysLogHandler` class, located in the :mod:`logging.handlers` module, supports sending logging messages to a remote or local Unix syslog. .. class:: SysLogHandler(address=('localhost', SYSLOG_UDP_PORT), facility=LOG_USER, socktype=socket.SOCK_DGRAM) Returns a new instance of the :class:`SysLogHandler` class intended to communicate with a remote Unix machine whose address is given by *address* in the form of a ``(host, port)`` tuple. If *address* is not specified, ``('localhost', 514)`` is used. The address is used to open a socket. An alternative to providing a ``(host, port)`` tuple is providing an address as a string, for example "/dev/log". In this case, a Unix domain socket is used to send the message to the syslog. If *facility* is not specified, :const:`LOG_USER` is used. The type of socket opened depends on the *socktype* argument, which defaults to :const:`socket.SOCK_DGRAM` and thus opens a UDP socket. To open a TCP socket (for use with the newer syslog daemons such as rsyslog), specify a value of :const:`socket.SOCK_STREAM`. Note that if your server is not listening on UDP port 514, :class:`SysLogHandler` may appear not to work. In that case, check what address you should be using for a domain socket - it's system dependent. For example, on Linux it's usually "/dev/log" but on OS/X it's "/var/run/syslog". You'll need to check your platform and use the appropriate address (you may need to do this check at runtime if your application needs to run on several platforms). On Windows, you pretty much have to use the UDP option. .. versionchanged:: 3.2 *socktype* was added. .. method:: close() Closes the socket to the remote host. .. method:: emit(record) The record is formatted, and then sent to the syslog server. If exception information is present, it is *not* sent to the server. .. method:: encodePriority(facility, priority) Encodes the facility and priority into an integer. You can pass in strings or integers - if strings are passed, internal mapping dictionaries are used to convert them to integers. The symbolic ``LOG_`` values are defined in :class:`SysLogHandler` and mirror the values defined in the ``sys/syslog.h`` header file. **Priorities** +--------------------------+---------------+ | Name (string) | Symbolic value| +==========================+===============+ | ``alert`` | LOG_ALERT | +--------------------------+---------------+ | ``crit`` or ``critical`` | LOG_CRIT | +--------------------------+---------------+ | ``debug`` | LOG_DEBUG | +--------------------------+---------------+ | ``emerg`` or ``panic`` | LOG_EMERG | +--------------------------+---------------+ | ``err`` or ``error`` | LOG_ERR | +--------------------------+---------------+ | ``info`` | LOG_INFO | +--------------------------+---------------+ | ``notice`` | LOG_NOTICE | +--------------------------+---------------+ | ``warn`` or ``warning`` | LOG_WARNING | +--------------------------+---------------+ **Facilities** +---------------+---------------+ | Name (string) | Symbolic value| +===============+===============+ | ``auth`` | LOG_AUTH | +---------------+---------------+ | ``authpriv`` | LOG_AUTHPRIV | +---------------+---------------+ | ``cron`` | LOG_CRON | +---------------+---------------+ | ``daemon`` | LOG_DAEMON | +---------------+---------------+ | ``ftp`` | LOG_FTP | +---------------+---------------+ | ``kern`` | LOG_KERN | +---------------+---------------+ | ``lpr`` | LOG_LPR | +---------------+---------------+ | ``mail`` | LOG_MAIL | +---------------+---------------+ | ``news`` | LOG_NEWS | +---------------+---------------+ | ``syslog`` | LOG_SYSLOG | +---------------+---------------+ | ``user`` | LOG_USER | +---------------+---------------+ | ``uucp`` | LOG_UUCP | +---------------+---------------+ | ``local0`` | LOG_LOCAL0 | +---------------+---------------+ | ``local1`` | LOG_LOCAL1 | +---------------+---------------+ | ``local2`` | LOG_LOCAL2 | +---------------+---------------+ | ``local3`` | LOG_LOCAL3 | +---------------+---------------+ | ``local4`` | LOG_LOCAL4 | +---------------+---------------+ | ``local5`` | LOG_LOCAL5 | +---------------+---------------+ | ``local6`` | LOG_LOCAL6 | +---------------+---------------+ | ``local7`` | LOG_LOCAL7 | +---------------+---------------+ .. method:: mapPriority(levelname) Maps a logging level name to a syslog priority name. You may need to override this if you are using custom levels, or if the default algorithm is not suitable for your needs. The default algorithm maps ``DEBUG``, ``INFO``, ``WARNING``, ``ERROR`` and ``CRITICAL`` to the equivalent syslog names, and all other level names to "warning". .. _nt-eventlog-handler: NTEventLogHandler ^^^^^^^^^^^^^^^^^ The :class:`NTEventLogHandler` class, located in the :mod:`logging.handlers` module, supports sending logging messages to a local Windows NT, Windows 2000 or Windows XP event log. Before you can use it, you need Mark Hammond's Win32 extensions for Python installed. .. class:: NTEventLogHandler(appname, dllname=None, logtype='Application') Returns a new instance of the :class:`NTEventLogHandler` class. The *appname* is used to define the application name as it appears in the event log. An appropriate registry entry is created using this name. The *dllname* should give the fully qualified pathname of a .dll or .exe which contains message definitions to hold in the log (if not specified, ``'win32service.pyd'`` is used - this is installed with the Win32 extensions and contains some basic placeholder message definitions. Note that use of these placeholders will make your event logs big, as the entire message source is held in the log. If you want slimmer logs, you have to pass in the name of your own .dll or .exe which contains the message definitions you want to use in the event log). The *logtype* is one of ``'Application'``, ``'System'`` or ``'Security'``, and defaults to ``'Application'``. .. method:: close() At this point, you can remove the application name from the registry as a source of event log entries. However, if you do this, you will not be able to see the events as you intended in the Event Log Viewer - it needs to be able to access the registry to get the .dll name. The current version does not do this. .. method:: emit(record) Determines the message ID, event category and event type, and then logs the message in the NT event log. .. method:: getEventCategory(record) Returns the event category for the record. Override this if you want to specify your own categories. This version returns 0. .. method:: getEventType(record) Returns the event type for the record. Override this if you want to specify your own types. This version does a mapping using the handler's typemap attribute, which is set up in :meth:`__init__` to a dictionary which contains mappings for :const:`DEBUG`, :const:`INFO`, :const:`WARNING`, :const:`ERROR` and :const:`CRITICAL`. If you are using your own levels, you will either need to override this method or place a suitable dictionary in the handler's *typemap* attribute. .. method:: getMessageID(record) Returns the message ID for the record. If you are using your own messages, you could do this by having the *msg* passed to the logger being an ID rather than a format string. Then, in here, you could use a dictionary lookup to get the message ID. This version returns 1, which is the base message ID in :file:`win32service.pyd`. .. _smtp-handler: SMTPHandler ^^^^^^^^^^^ The :class:`SMTPHandler` class, located in the :mod:`logging.handlers` module, supports sending logging messages to an email address via SMTP. .. class:: SMTPHandler(mailhost, fromaddr, toaddrs, subject, credentials=None) Returns a new instance of the :class:`SMTPHandler` class. The instance is initialized with the from and to addresses and subject line of the email. The *toaddrs* should be a list of strings. To specify a non-standard SMTP port, use the (host, port) tuple format for the *mailhost* argument. If you use a string, the standard SMTP port is used. If your SMTP server requires authentication, you can specify a (username, password) tuple for the *credentials* argument. .. method:: emit(record) Formats the record and sends it to the specified addressees. .. method:: getSubject(record) If you want to specify a subject line which is record-dependent, override this method. .. _memory-handler: MemoryHandler ^^^^^^^^^^^^^ The :class:`MemoryHandler` class, located in the :mod:`logging.handlers` module, supports buffering of logging records in memory, periodically flushing them to a :dfn:`target` handler. Flushing occurs whenever the buffer is full, or when an event of a certain severity or greater is seen. :class:`MemoryHandler` is a subclass of the more general :class:`BufferingHandler`, which is an abstract class. This buffers logging records in memory. Whenever each record is added to the buffer, a check is made by calling :meth:`shouldFlush` to see if the buffer should be flushed. If it should, then :meth:`flush` is expected to do the needful. .. class:: BufferingHandler(capacity) Initializes the handler with a buffer of the specified capacity. .. method:: emit(record) Appends the record to the buffer. If :meth:`shouldFlush` returns true, calls :meth:`flush` to process the buffer. .. method:: flush() You can override this to implement custom flushing behavior. This version just zaps the buffer to empty. .. method:: shouldFlush(record) Returns true if the buffer is up to capacity. This method can be overridden to implement custom flushing strategies. .. class:: MemoryHandler(capacity, flushLevel=ERROR, target=None) Returns a new instance of the :class:`MemoryHandler` class. The instance is initialized with a buffer size of *capacity*. If *flushLevel* is not specified, :const:`ERROR` is used. If no *target* is specified, the target will need to be set using :meth:`setTarget` before this handler does anything useful. .. method:: close() Calls :meth:`flush`, sets the target to :const:`None` and clears the buffer. .. method:: flush() For a :class:`MemoryHandler`, flushing means just sending the buffered records to the target, if there is one. The buffer is also cleared when this happens. Override if you want different behavior. .. method:: setTarget(target) Sets the target handler for this handler. .. method:: shouldFlush(record) Checks for buffer full or a record at the *flushLevel* or higher. .. _http-handler: HTTPHandler ^^^^^^^^^^^ The :class:`HTTPHandler` class, located in the :mod:`logging.handlers` module, supports sending logging messages to a Web server, using either ``GET`` or ``POST`` semantics. .. class:: HTTPHandler(host, url, method='GET', secure=False, credentials=None) Returns a new instance of the :class:`HTTPHandler` class. The *host* can be of the form ``host:port``, should you need to use a specific port number. If no *method* is specified, ``GET`` is used. If *secure* is True, an HTTPS connection will be used. If *credentials* is specified, it should be a 2-tuple consisting of userid and password, which will be placed in an HTTP 'Authorization' header using Basic authentication. If you specify credentials, you should also specify secure=True so that your userid and password are not passed in cleartext across the wire. .. method:: emit(record) Sends the record to the Web server as a percent-encoded dictionary. .. _queue-handler: QueueHandler ^^^^^^^^^^^^ .. versionadded:: 3.2 The :class:`QueueHandler` class, located in the :mod:`logging.handlers` module, supports sending logging messages to a queue, such as those implemented in the :mod:`queue` or :mod:`multiprocessing` modules. Along with the :class:`QueueListener` class, :class:`QueueHandler` can be used to let handlers do their work on a separate thread from the one which does the logging. This is important in Web applications and also other service applications where threads servicing clients need to respond as quickly as possible, while any potentially slow operations (such as sending an email via :class:`SMTPHandler`) are done on a separate thread. .. class:: QueueHandler(queue) Returns a new instance of the :class:`QueueHandler` class. The instance is initialized with the queue to send messages to. The queue can be any queue- like object; it's used as-is by the :meth:`enqueue` method, which needs to know how to send messages to it. .. method:: emit(record) Enqueues the result of preparing the LogRecord. .. method:: prepare(record) Prepares a record for queuing. The object returned by this method is enqueued. The base implementation formats the record to merge the message and arguments, and removes unpickleable items from the record in-place. You might want to override this method if you want to convert the record to a dict or JSON string, or send a modified copy of the record while leaving the original intact. .. method:: enqueue(record) Enqueues the record on the queue using ``put_nowait()``; you may want to override this if you want to use blocking behaviour, or a timeout, or a customised queue implementation. .. queue-listener: QueueListener ^^^^^^^^^^^^^ .. versionadded:: 3.2 The :class:`QueueListener` class, located in the :mod:`logging.handlers` module, supports receiving logging messages from a queue, such as those implemented in the :mod:`queue` or :mod:`multiprocessing` modules. The messages are received from a queue in an internal thread and passed, on the same thread, to one or more handlers for processing. Along with the :class:`QueueHandler` class, :class:`QueueListener` can be used to let handlers do their work on a separate thread from the one which does the logging. This is important in Web applications and also other service applications where threads servicing clients need to respond as quickly as possible, while any potentially slow operations (such as sending an email via :class:`SMTPHandler`) are done on a separate thread. .. class:: QueueListener(queue, *handlers) Returns a new instance of the :class:`QueueListener` class. The instance is initialized with the queue to send messages to and a list of handlers which will handle entries placed on the queue. The queue can be any queue- like object; it's passed as-is to the :meth:`dequeue` method, which needs to know how to get messages from it. .. method:: dequeue(block) Dequeues a record and return it, optionally blocking. The base implementation uses ``get()``. You may want to override this method if you want to use timeouts or work with custom queue implementations. .. method:: prepare(record) Prepare a record for handling. This implementation just returns the passed-in record. You may want to override this method if you need to do any custom marshalling or manipulation of the record before passing it to the handlers. .. method:: handle(record) Handle a record. This just loops through the handlers offering them the record to handle. The actual object passed to the handlers is that which is returned from :meth:`prepare`. .. method:: start() Starts the listener. This starts up a background thread to monitor the queue for LogRecords to process. .. method:: stop() Stops the listener. This asks the thread to terminate, and then waits for it to do so. Note that if you don't call this before your application exits, there may be some records still left on the queue, which won't be processed. .. _zeromq-handlers: Subclassing QueueHandler ^^^^^^^^^^^^^^^^^^^^^^^^ You can use a :class:`QueueHandler` subclass to send messages to other kinds of queues, for example a ZeroMQ "publish" socket. In the example below,the socket is created separately and passed to the handler (as its 'queue'):: import zmq # using pyzmq, the Python binding for ZeroMQ import json # for serializing records portably ctx = zmq.Context() sock = zmq.Socket(ctx, zmq.PUB) # or zmq.PUSH, or other suitable value sock.bind('tcp://*:5556') # or wherever class ZeroMQSocketHandler(QueueHandler): def enqueue(self, record): data = json.dumps(record.__dict__) self.queue.send(data) handler = ZeroMQSocketHandler(sock) Of course there are other ways of organizing this, for example passing in the data needed by the handler to create the socket:: class ZeroMQSocketHandler(QueueHandler): def __init__(self, uri, socktype=zmq.PUB, ctx=None): self.ctx = ctx or zmq.Context() socket = zmq.Socket(self.ctx, socktype) socket.bind(uri) QueueHandler.__init__(self, socket) def enqueue(self, record): data = json.dumps(record.__dict__) self.queue.send(data) def close(self): self.queue.close() Subclassing QueueListener ^^^^^^^^^^^^^^^^^^^^^^^^^ You can also subclass :class:`QueueListener` to get messages from other kinds of queues, for example a ZeroMQ "subscribe" socket. Here's an example:: class ZeroMQSocketListener(QueueListener): def __init__(self, uri, *handlers, **kwargs): self.ctx = kwargs.get('ctx') or zmq.Context() socket = zmq.Socket(self.ctx, zmq.SUB) socket.setsockopt(zmq.SUBSCRIBE, '') # subscribe to everything socket.connect(uri) def dequeue(self): msg = self.queue.recv() return logging.makeLogRecord(json.loads(msg)) .. _formatter-objects: Formatter Objects ----------------- .. currentmodule:: logging :class:`Formatter`\ s have the following attributes and methods. They are responsible for converting a :class:`LogRecord` to (usually) a string which can be interpreted by either a human or an external system. The base :class:`Formatter` allows a formatting string to be specified. If none is supplied, the default value of ``'%(message)s'`` is used. A Formatter can be initialized with a format string which makes use of knowledge of the :class:`LogRecord` attributes - such as the default value mentioned above making use of the fact that the user's message and arguments are pre-formatted into a :class:`LogRecord`'s *message* attribute. This format string contains standard Python %-style mapping keys. See section :ref:`old-string-formatting` for more information on string formatting. Currently, the useful mapping keys in a :class:`LogRecord` are: +-------------------------+-----------------------------------------------+ | Format | Description | +=========================+===============================================+ | ``%(name)s`` | Name of the logger (logging channel). | +-------------------------+-----------------------------------------------+ | ``%(levelno)s`` | Numeric logging level for the message | | | (:const:`DEBUG`, :const:`INFO`, | | | :const:`WARNING`, :const:`ERROR`, | | | :const:`CRITICAL`). | +-------------------------+-----------------------------------------------+ | ``%(levelname)s`` | Text logging level for the message | | | (``'DEBUG'``, ``'INFO'``, ``'WARNING'``, | | | ``'ERROR'``, ``'CRITICAL'``). | +-------------------------+-----------------------------------------------+ | ``%(pathname)s`` | Full pathname of the source file where the | | | logging call was issued (if available). | +-------------------------+-----------------------------------------------+ | ``%(filename)s`` | Filename portion of pathname. | +-------------------------+-----------------------------------------------+ | ``%(module)s`` | Module (name portion of filename). | +-------------------------+-----------------------------------------------+ | ``%(funcName)s`` | Name of function containing the logging call. | +-------------------------+-----------------------------------------------+ | ``%(lineno)d`` | Source line number where the logging call was | | | issued (if available). | +-------------------------+-----------------------------------------------+ | ``%(created)f`` | Time when the :class:`LogRecord` was created | | | (as returned by :func:`time.time`). | +-------------------------+-----------------------------------------------+ | ``%(relativeCreated)d`` | Time in milliseconds when the LogRecord was | | | created, relative to the time the logging | | | module was loaded. | +-------------------------+-----------------------------------------------+ | ``%(asctime)s`` | Human-readable time when the | | | :class:`LogRecord` was created. By default | | | this is of the form "2003-07-08 16:49:45,896" | | | (the numbers after the comma are millisecond | | | portion of the time). | +-------------------------+-----------------------------------------------+ | ``%(msecs)d`` | Millisecond portion of the time when the | | | :class:`LogRecord` was created. | +-------------------------+-----------------------------------------------+ | ``%(thread)d`` | Thread ID (if available). | +-------------------------+-----------------------------------------------+ | ``%(threadName)s`` | Thread name (if available). | +-------------------------+-----------------------------------------------+ | ``%(process)d`` | Process ID (if available). | +-------------------------+-----------------------------------------------+ | ``%(processName)s`` | Process name (if available). | +-------------------------+-----------------------------------------------+ | ``%(message)s`` | The logged message, computed as ``msg % | | | args``. | +-------------------------+-----------------------------------------------+ .. class:: Formatter(fmt=None, datefmt=None) Returns a new instance of the :class:`Formatter` class. The instance is initialized with a format string for the message as a whole, as well as a format string for the date/time portion of a message. If no *fmt* is specified, ``'%(message)s'`` is used. If no *datefmt* is specified, the ISO8601 date format is used. .. method:: format(record) The record's attribute dictionary is used as the operand to a string formatting operation. Returns the resulting string. Before formatting the dictionary, a couple of preparatory steps are carried out. The *message* attribute of the record is computed using *msg* % *args*. If the formatting string contains ``'(asctime)'``, :meth:`formatTime` is called to format the event time. If there is exception information, it is formatted using :meth:`formatException` and appended to the message. Note that the formatted exception information is cached in attribute *exc_text*. This is useful because the exception information can be pickled and sent across the wire, but you should be careful if you have more than one :class:`Formatter` subclass which customizes the formatting of exception information. In this case, you will have to clear the cached value after a formatter has done its formatting, so that the next formatter to handle the event doesn't use the cached value but recalculates it afresh. If stack information is available, it's appended after the exception information, using :meth:`formatStack` to transform it if necessary. .. method:: formatTime(record, datefmt=None) This method should be called from :meth:`format` by a formatter which wants to make use of a formatted time. This method can be overridden in formatters to provide for any specific requirement, but the basic behavior is as follows: if *datefmt* (a string) is specified, it is used with :func:`time.strftime` to format the creation time of the record. Otherwise, the ISO8601 format is used. The resulting string is returned. .. method:: formatException(exc_info) Formats the specified exception information (a standard exception tuple as returned by :func:`sys.exc_info`) as a string. This default implementation just uses :func:`traceback.print_exception`. The resulting string is returned. .. method:: formatStack(stack_info) Formats the specified stack information (a string as returned by :func:`traceback.print_stack`, but with the last newline removed) as a string. This default implementation just returns the input value. .. _filter: Filter Objects -------------- ``Filters`` can be used by ``Handlers`` and ``Loggers`` for more sophisticated filtering than is provided by levels. The base filter class only allows events which are below a certain point in the logger hierarchy. For example, a filter initialized with "A.B" will allow events logged by loggers "A.B", "A.B.C", "A.B.C.D", "A.B.D" etc. but not "A.BB", "B.A.B" etc. If initialized with the empty string, all events are passed. .. class:: Filter(name='') Returns an instance of the :class:`Filter` class. If *name* is specified, it names a logger which, together with its children, will have its events allowed through the filter. If *name* is the empty string, allows every event. .. method:: filter(record) Is the specified record to be logged? Returns zero for no, nonzero for yes. If deemed appropriate, the record may be modified in-place by this method. Note that filters attached to handlers are consulted whenever an event is emitted by the handler, whereas filters attached to loggers are consulted whenever an event is logged to the handler (using :meth:`debug`, :meth:`info`, etc.) This means that events which have been generated by descendant loggers will not be filtered by a logger's filter setting, unless the filter has also been applied to those descendant loggers. You don't actually need to subclass ``Filter``: you can pass any instance which has a ``filter`` method with the same semantics. .. versionchanged:: 3.2 You don't need to create specialized ``Filter`` classes, or use other classes with a ``filter`` method: you can use a function (or other callable) as a filter. The filtering logic will check to see if the filter object has a ``filter`` attribute: if it does, it's assumed to be a ``Filter`` and its :meth:`~Filter.filter` method is called. Otherwise, it's assumed to be a callable and called with the record as the single parameter. The returned value should conform to that returned by :meth:`~Filter.filter`. Other uses for filters ^^^^^^^^^^^^^^^^^^^^^^ Although filters are used primarily to filter records based on more sophisticated criteria than levels, they get to see every record which is processed by the handler or logger they're attached to: this can be useful if you want to do things like counting how many records were processed by a particular logger or handler, or adding, changing or removing attributes in the LogRecord being processed. Obviously changing the LogRecord needs to be done with some care, but it does allow the injection of contextual information into logs (see :ref:`filters-contextual`). .. _log-record: LogRecord Objects ----------------- :class:`LogRecord` instances are created automatically by the :class:`Logger` every time something is logged, and can be created manually via :func:`makeLogRecord` (for example, from a pickled event received over the wire). .. class:: LogRecord(name, lvl, pathname, lineno, msg, args, exc_info, func=None, sinfo=None) Contains all the information pertinent to the event being logged. The primary information is passed in :attr:`msg` and :attr:`args`, which are combined using ``msg % args`` to create the :attr:`message` field of the record. .. attribute:: args Tuple of arguments to be used in formatting :attr:`msg`. .. attribute:: exc_info Exception tuple (à la :func:`sys.exc_info`) or ``None`` if no exception information is available. .. attribute:: func Name of the function of origin (i.e. in which the logging call was made). .. attribute:: lineno Line number in the source file of origin. .. attribute:: lvl Numeric logging level. .. attribute:: message Bound to the result of :meth:`getMessage` when :meth:`Formatter.format(record)` is invoked. .. attribute:: msg User-supplied :ref:`format string` or arbitrary object (see :ref:`arbitrary-object-messages`) used in :meth:`getMessage`. .. attribute:: name Name of the logger that emitted the record. .. attribute:: pathname Absolute pathname of the source file of origin. .. attribute:: stack_info Stack frame information (where available) from the bottom of the stack in the current thread, up to and including the stack frame of the logging call which resulted in the creation of this record. .. method:: getMessage() Returns the message for this :class:`LogRecord` instance after merging any user-supplied arguments with the message. If the user-supplied message argument to the logging call is not a string, :func:`str` is called on it to convert it to a string. This allows use of user-defined classes as messages, whose ``__str__`` method can return the actual format string to be used. .. versionchanged:: 3.2 The creation of a ``LogRecord`` has been made more configurable by providing a factory which is used to create the record. The factory can be set using :func:`getLogRecordFactory` and :func:`setLogRecordFactory` (see this for the factory's signature). This functionality can be used to inject your own values into a LogRecord at creation time. You can use the following pattern:: old_factory = logging.getLogRecordFactory() def record_factory(*args, **kwargs): record = old_factory(*args, **kwargs) record.custom_attribute = 0xdecafbad return record logging.setLogRecordFactory(record_factory) With this pattern, multiple factories could be chained, and as long as they don't overwrite each other's attributes or unintentionally overwrite the standard attributes listed above, there should be no surprises. .. _logger-adapter: LoggerAdapter Objects --------------------- :class:`LoggerAdapter` instances are used to conveniently pass contextual information into logging calls. For a usage example , see the section on :ref:`adding contextual information to your logging output `. .. class:: LoggerAdapter(logger, extra) Returns an instance of :class:`LoggerAdapter` initialized with an underlying :class:`Logger` instance and a dict-like object. .. method:: process(msg, kwargs) Modifies the message and/or keyword arguments passed to a logging call in order to insert contextual information. This implementation takes the object passed as *extra* to the constructor and adds it to *kwargs* using key 'extra'. The return value is a (*msg*, *kwargs*) tuple which has the (possibly modified) versions of the arguments passed in. In addition to the above, :class:`LoggerAdapter` supports the following methods of :class:`Logger`, i.e. :meth:`debug`, :meth:`info`, :meth:`warning`, :meth:`error`, :meth:`exception`, :meth:`critical`, :meth:`log`, :meth:`isEnabledFor`, :meth:`getEffectiveLevel`, :meth:`setLevel`, :meth:`hasHandlers`. These methods have the same signatures as their counterparts in :class:`Logger`, so you can use the two types of instances interchangeably. .. versionchanged:: 3.2 The :meth:`isEnabledFor`, :meth:`getEffectiveLevel`, :meth:`setLevel` and :meth:`hasHandlers` methods were added to :class:`LoggerAdapter`. These methods delegate to the underlying logger. Thread Safety ------------- The logging module is intended to be thread-safe without any special work needing to be done by its clients. It achieves this though using threading locks; there is one lock to serialize access to the module's shared data, and each handler also creates a lock to serialize access to its underlying I/O. If you are implementing asynchronous signal handlers using the :mod:`signal` module, you may not be able to use logging from within such handlers. This is because lock implementations in the :mod:`threading` module are not always re-entrant, and so cannot be invoked from such signal handlers. Integration with the warnings module ------------------------------------ The :func:`captureWarnings` function can be used to integrate :mod:`logging` with the :mod:`warnings` module. .. function:: captureWarnings(capture) This function is used to turn the capture of warnings by logging on and off. If *capture* is ``True``, warnings issued by the :mod:`warnings` module will be redirected to the logging system. Specifically, a warning will be formatted using :func:`warnings.formatwarning` and the resulting string logged to a logger named "py.warnings" with a severity of `WARNING`. If *capture* is ``False``, the redirection of warnings to the logging system will stop, and warnings will be redirected to their original destinations (i.e. those in effect before `captureWarnings(True)` was called). Configuration ------------- .. _logging-config-api: Configuration functions ^^^^^^^^^^^^^^^^^^^^^^^ The following functions configure the logging module. They are located in the :mod:`logging.config` module. Their use is optional --- you can configure the logging module using these functions or by making calls to the main API (defined in :mod:`logging` itself) and defining handlers which are declared either in :mod:`logging` or :mod:`logging.handlers`. .. function:: dictConfig(config) Takes the logging configuration from a dictionary. The contents of this dictionary are described in :ref:`logging-config-dictschema` below. If an error is encountered during configuration, this function will raise a :exc:`ValueError`, :exc:`TypeError`, :exc:`AttributeError` or :exc:`ImportError` with a suitably descriptive message. The following is a (possibly incomplete) list of conditions which will raise an error: * A ``level`` which is not a string or which is a string not corresponding to an actual logging level. * A ``propagate`` value which is not a boolean. * An id which does not have a corresponding destination. * A non-existent handler id found during an incremental call. * An invalid logger name. * Inability to resolve to an internal or external object. Parsing is performed by the :class:`DictConfigurator` class, whose constructor is passed the dictionary used for configuration, and has a :meth:`configure` method. The :mod:`logging.config` module has a callable attribute :attr:`dictConfigClass` which is initially set to :class:`DictConfigurator`. You can replace the value of :attr:`dictConfigClass` with a suitable implementation of your own. :func:`dictConfig` calls :attr:`dictConfigClass` passing the specified dictionary, and then calls the :meth:`configure` method on the returned object to put the configuration into effect:: def dictConfig(config): dictConfigClass(config).configure() For example, a subclass of :class:`DictConfigurator` could call ``DictConfigurator.__init__()`` in its own :meth:`__init__()`, then set up custom prefixes which would be usable in the subsequent :meth:`configure` call. :attr:`dictConfigClass` would be bound to this new subclass, and then :func:`dictConfig` could be called exactly as in the default, uncustomized state. .. function:: fileConfig(fname[, defaults]) Reads the logging configuration from a :mod:`configparser`\-format file named *fname*. This function can be called several times from an application, allowing an end user to select from various pre-canned configurations (if the developer provides a mechanism to present the choices and load the chosen configuration). Defaults to be passed to the ConfigParser can be specified in the *defaults* argument. .. function:: listen(port=DEFAULT_LOGGING_CONFIG_PORT) Starts up a socket server on the specified port, and listens for new configurations. If no port is specified, the module's default :const:`DEFAULT_LOGGING_CONFIG_PORT` is used. Logging configurations will be sent as a file suitable for processing by :func:`fileConfig`. Returns a :class:`Thread` instance on which you can call :meth:`start` to start the server, and which you can :meth:`join` when appropriate. To stop the server, call :func:`stopListening`. To send a configuration to the socket, read in the configuration file and send it to the socket as a string of bytes preceded by a four-byte length string packed in binary using ``struct.pack('>L', n)``. .. function:: stopListening() Stops the listening server which was created with a call to :func:`listen`. This is typically called before calling :meth:`join` on the return value from :func:`listen`. .. _logging-config-dictschema: Configuration dictionary schema ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Describing a logging configuration requires listing the various objects to create and the connections between them; for example, you may create a handler named "console" and then say that the logger named "startup" will send its messages to the "console" handler. These objects aren't limited to those provided by the :mod:`logging` module because you might write your own formatter or handler class. The parameters to these classes may also need to include external objects such as ``sys.stderr``. The syntax for describing these objects and connections is defined in :ref:`logging-config-dict-connections` below. Dictionary Schema Details """"""""""""""""""""""""" The dictionary passed to :func:`dictConfig` must contain the following keys: * *version* - to be set to an integer value representing the schema version. The only valid value at present is 1, but having this key allows the schema to evolve while still preserving backwards compatibility. All other keys are optional, but if present they will be interpreted as described below. In all cases below where a 'configuring dict' is mentioned, it will be checked for the special ``'()'`` key to see if a custom instantiation is required. If so, the mechanism described in :ref:`logging-config-dict-userdef` below is used to create an instance; otherwise, the context is used to determine what to instantiate. * *formatters* - the corresponding value will be a dict in which each key is a formatter id and each value is a dict describing how to configure the corresponding Formatter instance. The configuring dict is searched for keys ``format`` and ``datefmt`` (with defaults of ``None``) and these are used to construct a :class:`logging.Formatter` instance. * *filters* - the corresponding value will be a dict in which each key is a filter id and each value is a dict describing how to configure the corresponding Filter instance. The configuring dict is searched for the key ``name`` (defaulting to the empty string) and this is used to construct a :class:`logging.Filter` instance. * *handlers* - the corresponding value will be a dict in which each key is a handler id and each value is a dict describing how to configure the corresponding Handler instance. The configuring dict is searched for the following keys: * ``class`` (mandatory). This is the fully qualified name of the handler class. * ``level`` (optional). The level of the handler. * ``formatter`` (optional). The id of the formatter for this handler. * ``filters`` (optional). A list of ids of the filters for this handler. All *other* keys are passed through as keyword arguments to the handler's constructor. For example, given the snippet:: handlers: console: class : logging.StreamHandler formatter: brief level : INFO filters: [allow_foo] stream : ext://sys.stdout file: class : logging.handlers.RotatingFileHandler formatter: precise filename: logconfig.log maxBytes: 1024 backupCount: 3 the handler with id ``console`` is instantiated as a :class:`logging.StreamHandler`, using ``sys.stdout`` as the underlying stream. The handler with id ``file`` is instantiated as a :class:`logging.handlers.RotatingFileHandler` with the keyword arguments ``filename='logconfig.log', maxBytes=1024, backupCount=3``. * *loggers* - the corresponding value will be a dict in which each key is a logger name and each value is a dict describing how to configure the corresponding Logger instance. The configuring dict is searched for the following keys: * ``level`` (optional). The level of the logger. * ``propagate`` (optional). The propagation setting of the logger. * ``filters`` (optional). A list of ids of the filters for this logger. * ``handlers`` (optional). A list of ids of the handlers for this logger. The specified loggers will be configured according to the level, propagation, filters and handlers specified. * *root* - this will be the configuration for the root logger. Processing of the configuration will be as for any logger, except that the ``propagate`` setting will not be applicable. * *incremental* - whether the configuration is to be interpreted as incremental to the existing configuration. This value defaults to ``False``, which means that the specified configuration replaces the existing configuration with the same semantics as used by the existing :func:`fileConfig` API. If the specified value is ``True``, the configuration is processed as described in the section on :ref:`logging-config-dict-incremental`. * *disable_existing_loggers* - whether any existing loggers are to be disabled. This setting mirrors the parameter of the same name in :func:`fileConfig`. If absent, this parameter defaults to ``True``. This value is ignored if *incremental* is ``True``. .. _logging-config-dict-incremental: Incremental Configuration """"""""""""""""""""""""" It is difficult to provide complete flexibility for incremental configuration. For example, because objects such as filters and formatters are anonymous, once a configuration is set up, it is not possible to refer to such anonymous objects when augmenting a configuration. Furthermore, there is not a compelling case for arbitrarily altering the object graph of loggers, handlers, filters, formatters at run-time, once a configuration is set up; the verbosity of loggers and handlers can be controlled just by setting levels (and, in the case of loggers, propagation flags). Changing the object graph arbitrarily in a safe way is problematic in a multi-threaded environment; while not impossible, the benefits are not worth the complexity it adds to the implementation. Thus, when the ``incremental`` key of a configuration dict is present and is ``True``, the system will completely ignore any ``formatters`` and ``filters`` entries, and process only the ``level`` settings in the ``handlers`` entries, and the ``level`` and ``propagate`` settings in the ``loggers`` and ``root`` entries. Using a value in the configuration dict lets configurations to be sent over the wire as pickled dicts to a socket listener. Thus, the logging verbosity of a long-running application can be altered over time with no need to stop and restart the application. .. _logging-config-dict-connections: Object connections """""""""""""""""" The schema describes a set of logging objects - loggers, handlers, formatters, filters - which are connected to each other in an object graph. Thus, the schema needs to represent connections between the objects. For example, say that, once configured, a particular logger has attached to it a particular handler. For the purposes of this discussion, we can say that the logger represents the source, and the handler the destination, of a connection between the two. Of course in the configured objects this is represented by the logger holding a reference to the handler. In the configuration dict, this is done by giving each destination object an id which identifies it unambiguously, and then using the id in the source object's configuration to indicate that a connection exists between the source and the destination object with that id. So, for example, consider the following YAML snippet:: formatters: brief: # configuration for formatter with id 'brief' goes here precise: # configuration for formatter with id 'precise' goes here handlers: h1: #This is an id # configuration of handler with id 'h1' goes here formatter: brief h2: #This is another id # configuration of handler with id 'h2' goes here formatter: precise loggers: foo.bar.baz: # other configuration for logger 'foo.bar.baz' handlers: [h1, h2] (Note: YAML used here because it's a little more readable than the equivalent Python source form for the dictionary.) The ids for loggers are the logger names which would be used programmatically to obtain a reference to those loggers, e.g. ``foo.bar.baz``. The ids for Formatters and Filters can be any string value (such as ``brief``, ``precise`` above) and they are transient, in that they are only meaningful for processing the configuration dictionary and used to determine connections between objects, and are not persisted anywhere when the configuration call is complete. The above snippet indicates that logger named ``foo.bar.baz`` should have two handlers attached to it, which are described by the handler ids ``h1`` and ``h2``. The formatter for ``h1`` is that described by id ``brief``, and the formatter for ``h2`` is that described by id ``precise``. .. _logging-config-dict-userdef: User-defined objects """""""""""""""""""" The schema supports user-defined objects for handlers, filters and formatters. (Loggers do not need to have different types for different instances, so there is no support in this configuration schema for user-defined logger classes.) Objects to be configured are described by dictionaries which detail their configuration. In some places, the logging system will be able to infer from the context how an object is to be instantiated, but when a user-defined object is to be instantiated, the system will not know how to do this. In order to provide complete flexibility for user-defined object instantiation, the user needs to provide a 'factory' - a callable which is called with a configuration dictionary and which returns the instantiated object. This is signalled by an absolute import path to the factory being made available under the special key ``'()'``. Here's a concrete example:: formatters: brief: format: '%(message)s' default: format: '%(asctime)s %(levelname)-8s %(name)-15s %(message)s' datefmt: '%Y-%m-%d %H:%M:%S' custom: (): my.package.customFormatterFactory bar: baz spam: 99.9 answer: 42 The above YAML snippet defines three formatters. The first, with id ``brief``, is a standard :class:`logging.Formatter` instance with the specified format string. The second, with id ``default``, has a longer format and also defines the time format explicitly, and will result in a :class:`logging.Formatter` initialized with those two format strings. Shown in Python source form, the ``brief`` and ``default`` formatters have configuration sub-dictionaries:: { 'format' : '%(message)s' } and:: { 'format' : '%(asctime)s %(levelname)-8s %(name)-15s %(message)s', 'datefmt' : '%Y-%m-%d %H:%M:%S' } respectively, and as these dictionaries do not contain the special key ``'()'``, the instantiation is inferred from the context: as a result, standard :class:`logging.Formatter` instances are created. The configuration sub-dictionary for the third formatter, with id ``custom``, is:: { '()' : 'my.package.customFormatterFactory', 'bar' : 'baz', 'spam' : 99.9, 'answer' : 42 } and this contains the special key ``'()'``, which means that user-defined instantiation is wanted. In this case, the specified factory callable will be used. If it is an actual callable it will be used directly - otherwise, if you specify a string (as in the example) the actual callable will be located using normal import mechanisms. The callable will be called with the **remaining** items in the configuration sub-dictionary as keyword arguments. In the above example, the formatter with id ``custom`` will be assumed to be returned by the call:: my.package.customFormatterFactory(bar='baz', spam=99.9, answer=42) The key ``'()'`` has been used as the special key because it is not a valid keyword parameter name, and so will not clash with the names of the keyword arguments used in the call. The ``'()'`` also serves as a mnemonic that the corresponding value is a callable. .. _logging-config-dict-externalobj: Access to external objects """""""""""""""""""""""""" There are times where a configuration needs to refer to objects external to the configuration, for example ``sys.stderr``. If the configuration dict is constructed using Python code, this is straightforward, but a problem arises when the configuration is provided via a text file (e.g. JSON, YAML). In a text file, there is no standard way to distinguish ``sys.stderr`` from the literal string ``'sys.stderr'``. To facilitate this distinction, the configuration system looks for certain special prefixes in string values and treat them specially. For example, if the literal string ``'ext://sys.stderr'`` is provided as a value in the configuration, then the ``ext://`` will be stripped off and the remainder of the value processed using normal import mechanisms. The handling of such prefixes is done in a way analogous to protocol handling: there is a generic mechanism to look for prefixes which match the regular expression ``^(?P[a-z]+)://(?P.*)$`` whereby, if the ``prefix`` is recognised, the ``suffix`` is processed in a prefix-dependent manner and the result of the processing replaces the string value. If the prefix is not recognised, then the string value will be left as-is. .. _logging-config-dict-internalobj: Access to internal objects """""""""""""""""""""""""" As well as external objects, there is sometimes also a need to refer to objects in the configuration. This will be done implicitly by the configuration system for things that it knows about. For example, the string value ``'DEBUG'`` for a ``level`` in a logger or handler will automatically be converted to the value ``logging.DEBUG``, and the ``handlers``, ``filters`` and ``formatter`` entries will take an object id and resolve to the appropriate destination object. However, a more generic mechanism is needed for user-defined objects which are not known to the :mod:`logging` module. For example, consider :class:`logging.handlers.MemoryHandler`, which takes a ``target`` argument which is another handler to delegate to. Since the system already knows about this class, then in the configuration, the given ``target`` just needs to be the object id of the relevant target handler, and the system will resolve to the handler from the id. If, however, a user defines a ``my.package.MyHandler`` which has an ``alternate`` handler, the configuration system would not know that the ``alternate`` referred to a handler. To cater for this, a generic resolution system allows the user to specify:: handlers: file: # configuration of file handler goes here custom: (): my.package.MyHandler alternate: cfg://handlers.file The literal string ``'cfg://handlers.file'`` will be resolved in an analogous way to strings with the ``ext://`` prefix, but looking in the configuration itself rather than the import namespace. The mechanism allows access by dot or by index, in a similar way to that provided by ``str.format``. Thus, given the following snippet:: handlers: email: class: logging.handlers.SMTPHandler mailhost: localhost fromaddr: my_app@domain.tld toaddrs: - support_team@domain.tld - dev_team@domain.tld subject: Houston, we have a problem. in the configuration, the string ``'cfg://handlers'`` would resolve to the dict with key ``handlers``, the string ``'cfg://handlers.email`` would resolve to the dict with key ``email`` in the ``handlers`` dict, and so on. The string ``'cfg://handlers.email.toaddrs[1]`` would resolve to ``'dev_team.domain.tld'`` and the string ``'cfg://handlers.email.toaddrs[0]'`` would resolve to the value ``'support_team@domain.tld'``. The ``subject`` value could be accessed using either ``'cfg://handlers.email.subject'`` or, equivalently, ``'cfg://handlers.email[subject]'``. The latter form only needs to be used if the key contains spaces or non-alphanumeric characters. If an index value consists only of decimal digits, access will be attempted using the corresponding integer value, falling back to the string value if needed. Given a string ``cfg://handlers.myhandler.mykey.123``, this will resolve to ``config_dict['handlers']['myhandler']['mykey']['123']``. If the string is specified as ``cfg://handlers.myhandler.mykey[123]``, the system will attempt to retrieve the value from ``config_dict['handlers']['myhandler']['mykey'][123]``, and fall back to ``config_dict['handlers']['myhandler']['mykey']['123']`` if that fails. .. _logging-config-fileformat: Configuration file format ^^^^^^^^^^^^^^^^^^^^^^^^^ The configuration file format understood by :func:`fileConfig` is based on :mod:`configparser` functionality. The file must contain sections called ``[loggers]``, ``[handlers]`` and ``[formatters]`` which identify by name the entities of each type which are defined in the file. For each such entity, there is a separate section which identifies how that entity is configured. Thus, for a logger named ``log01`` in the ``[loggers]`` section, the relevant configuration details are held in a section ``[logger_log01]``. Similarly, a handler called ``hand01`` in the ``[handlers]`` section will have its configuration held in a section called ``[handler_hand01]``, while a formatter called ``form01`` in the ``[formatters]`` section will have its configuration specified in a section called ``[formatter_form01]``. The root logger configuration must be specified in a section called ``[logger_root]``. Examples of these sections in the file are given below. :: [loggers] keys=root,log02,log03,log04,log05,log06,log07 [handlers] keys=hand01,hand02,hand03,hand04,hand05,hand06,hand07,hand08,hand09 [formatters] keys=form01,form02,form03,form04,form05,form06,form07,form08,form09 The root logger must specify a level and a list of handlers. An example of a root logger section is given below. :: [logger_root] level=NOTSET handlers=hand01 The ``level`` entry can be one of ``DEBUG, INFO, WARNING, ERROR, CRITICAL`` or ``NOTSET``. For the root logger only, ``NOTSET`` means that all messages will be logged. Level values are :func:`eval`\ uated in the context of the ``logging`` package's namespace. The ``handlers`` entry is a comma-separated list of handler names, which must appear in the ``[handlers]`` section. These names must appear in the ``[handlers]`` section and have corresponding sections in the configuration file. For loggers other than the root logger, some additional information is required. This is illustrated by the following example. :: [logger_parser] level=DEBUG handlers=hand01 propagate=1 qualname=compiler.parser The ``level`` and ``handlers`` entries are interpreted as for the root logger, except that if a non-root logger's level is specified as ``NOTSET``, the system consults loggers higher up the hierarchy to determine the effective level of the logger. The ``propagate`` entry is set to 1 to indicate that messages must propagate to handlers higher up the logger hierarchy from this logger, or 0 to indicate that messages are **not** propagated to handlers up the hierarchy. The ``qualname`` entry is the hierarchical channel name of the logger, that is to say the name used by the application to get the logger. Sections which specify handler configuration are exemplified by the following. :: [handler_hand01] class=StreamHandler level=NOTSET formatter=form01 args=(sys.stdout,) The ``class`` entry indicates the handler's class (as determined by :func:`eval` in the ``logging`` package's namespace). The ``level`` is interpreted as for loggers, and ``NOTSET`` is taken to mean "log everything". The ``formatter`` entry indicates the key name of the formatter for this handler. If blank, a default formatter (``logging._defaultFormatter``) is used. If a name is specified, it must appear in the ``[formatters]`` section and have a corresponding section in the configuration file. The ``args`` entry, when :func:`eval`\ uated in the context of the ``logging`` package's namespace, is the list of arguments to the constructor for the handler class. Refer to the constructors for the relevant handlers, or to the examples below, to see how typical entries are constructed. :: [handler_hand02] class=FileHandler level=DEBUG formatter=form02 args=('python.log', 'w') [handler_hand03] class=handlers.SocketHandler level=INFO formatter=form03 args=('localhost', handlers.DEFAULT_TCP_LOGGING_PORT) [handler_hand04] class=handlers.DatagramHandler level=WARN formatter=form04 args=('localhost', handlers.DEFAULT_UDP_LOGGING_PORT) [handler_hand05] class=handlers.SysLogHandler level=ERROR formatter=form05 args=(('localhost', handlers.SYSLOG_UDP_PORT), handlers.SysLogHandler.LOG_USER) [handler_hand06] class=handlers.NTEventLogHandler level=CRITICAL formatter=form06 args=('Python Application', '', 'Application') [handler_hand07] class=handlers.SMTPHandler level=WARN formatter=form07 args=('localhost', 'from@abc', ['user1@abc', 'user2@xyz'], 'Logger Subject') [handler_hand08] class=handlers.MemoryHandler level=NOTSET formatter=form08 target= args=(10, ERROR) [handler_hand09] class=handlers.HTTPHandler level=NOTSET formatter=form09 args=('localhost:9022', '/log', 'GET') Sections which specify formatter configuration are typified by the following. :: [formatter_form01] format=F1 %(asctime)s %(levelname)s %(message)s datefmt= class=logging.Formatter The ``format`` entry is the overall format string, and the ``datefmt`` entry is the :func:`strftime`\ -compatible date/time format string. If empty, the package substitutes ISO8601 format date/times, which is almost equivalent to specifying the date format string ``"%Y-%m-%d %H:%M:%S"``. The ISO8601 format also specifies milliseconds, which are appended to the result of using the above format string, with a comma separator. An example time in ISO8601 format is ``2003-01-23 00:29:50,411``. The ``class`` entry is optional. It indicates the name of the formatter's class (as a dotted module and class name.) This option is useful for instantiating a :class:`Formatter` subclass. Subclasses of :class:`Formatter` can present exception tracebacks in an expanded or condensed format. Configuration server example ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Here is an example of a module using the logging configuration server:: import logging import logging.config import time import os # read initial config file logging.config.fileConfig("logging.conf") # create and start listener on port 9999 t = logging.config.listen(9999) t.start() logger = logging.getLogger("simpleExample") try: # loop through logging calls to see the difference # new configurations make, until Ctrl+C is pressed while True: logger.debug("debug message") logger.info("info message") logger.warn("warn message") logger.error("error message") logger.critical("critical message") time.sleep(5) except KeyboardInterrupt: # cleanup logging.config.stopListening() t.join() And here is a script that takes a filename and sends that file to the server, properly preceded with the binary-encoded length, as the new logging configuration:: #!/usr/bin/env python import socket, sys, struct data_to_send = open(sys.argv[1], "r").read() HOST = 'localhost' PORT = 9999 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) print("connecting...") s.connect((HOST, PORT)) print("sending config...") s.send(struct.pack(">L", len(data_to_send))) s.send(data_to_send) s.close() print("complete") More examples ------------- Multiple handlers and formatters ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Loggers are plain Python objects. The :func:`addHandler` method has no minimum or maximum quota for the number of handlers you may add. Sometimes it will be beneficial for an application to log all messages of all severities to a text file while simultaneously logging errors or above to the console. To set this up, simply configure the appropriate handlers. The logging calls in the application code will remain unchanged. Here is a slight modification to the previous simple module-based configuration example:: import logging logger = logging.getLogger("simple_example") logger.setLevel(logging.DEBUG) # create file handler which logs even debug messages fh = logging.FileHandler("spam.log") fh.setLevel(logging.DEBUG) # create console handler with a higher log level ch = logging.StreamHandler() ch.setLevel(logging.ERROR) # create formatter and add it to the handlers formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s") ch.setFormatter(formatter) fh.setFormatter(formatter) # add the handlers to logger logger.addHandler(ch) logger.addHandler(fh) # "application" code logger.debug("debug message") logger.info("info message") logger.warn("warn message") logger.error("error message") logger.critical("critical message") Notice that the "application" code does not care about multiple handlers. All that changed was the addition and configuration of a new handler named *fh*. The ability to create new handlers with higher- or lower-severity filters can be very helpful when writing and testing an application. Instead of using many ``print`` statements for debugging, use ``logger.debug``: Unlike the print statements, which you will have to delete or comment out later, the logger.debug statements can remain intact in the source code and remain dormant until you need them again. At that time, the only change that needs to happen is to modify the severity level of the logger and/or handler to debug. Using logging in multiple modules ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ It was mentioned above that multiple calls to ``logging.getLogger('someLogger')`` return a reference to the same logger object. This is true not only within the same module, but also across modules as long as it is in the same Python interpreter process. It is true for references to the same object; additionally, application code can define and configure a parent logger in one module and create (but not configure) a child logger in a separate module, and all logger calls to the child will pass up to the parent. Here is a main module:: import logging import auxiliary_module # create logger with "spam_application" logger = logging.getLogger("spam_application") logger.setLevel(logging.DEBUG) # create file handler which logs even debug messages fh = logging.FileHandler("spam.log") fh.setLevel(logging.DEBUG) # create console handler with a higher log level ch = logging.StreamHandler() ch.setLevel(logging.ERROR) # create formatter and add it to the handlers formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s") fh.setFormatter(formatter) ch.setFormatter(formatter) # add the handlers to the logger logger.addHandler(fh) logger.addHandler(ch) logger.info("creating an instance of auxiliary_module.Auxiliary") a = auxiliary_module.Auxiliary() logger.info("created an instance of auxiliary_module.Auxiliary") logger.info("calling auxiliary_module.Auxiliary.do_something") a.do_something() logger.info("finished auxiliary_module.Auxiliary.do_something") logger.info("calling auxiliary_module.some_function()") auxiliary_module.some_function() logger.info("done with auxiliary_module.some_function()") Here is the auxiliary module:: import logging # create logger module_logger = logging.getLogger("spam_application.auxiliary") class Auxiliary: def __init__(self): self.logger = logging.getLogger("spam_application.auxiliary.Auxiliary") self.logger.info("creating an instance of Auxiliary") def do_something(self): self.logger.info("doing something") a = 1 + 1 self.logger.info("done doing something") def some_function(): module_logger.info("received a call to \"some_function\"") The output looks like this:: 2005-03-23 23:47:11,663 - spam_application - INFO - creating an instance of auxiliary_module.Auxiliary 2005-03-23 23:47:11,665 - spam_application.auxiliary.Auxiliary - INFO - creating an instance of Auxiliary 2005-03-23 23:47:11,665 - spam_application - INFO - created an instance of auxiliary_module.Auxiliary 2005-03-23 23:47:11,668 - spam_application - INFO - calling auxiliary_module.Auxiliary.do_something 2005-03-23 23:47:11,668 - spam_application.auxiliary.Auxiliary - INFO - doing something 2005-03-23 23:47:11,669 - spam_application.auxiliary.Auxiliary - INFO - done doing something 2005-03-23 23:47:11,670 - spam_application - INFO - finished auxiliary_module.Auxiliary.do_something 2005-03-23 23:47:11,671 - spam_application - INFO - calling auxiliary_module.some_function() 2005-03-23 23:47:11,672 - spam_application.auxiliary - INFO - received a call to "some_function" 2005-03-23 23:47:11,673 - spam_application - INFO - done with auxiliary_module.some_function()