Added section about adding contextual information to log output.

Doc/library/logging.rst

@@ -1137,6 +1137,52 @@ This example uses console and file handlers, but you can use any number and
 combination of handlers you choose.
 
 
+.. _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.
+
+There are a number of other ways you can pass contextual information to be
+output along with logging event information.
+
+* Use an adapter class which has access to the contextual information and
+  which defines methods :meth:`debug`, :meth:`info` etc. with the same
+  signatures as used by :class:`Logger`. You instantiate the adapter with a
+  name, which will be used to create an underlying :class:`Logger` with that
+  name. In each adpater method, the passed-in message is modified to include
+  whatever contextual information you want.
+
+* Use something other than a string to pass the message. Although normally
+  the first argument to a logger method such as :meth:`debug`, :meth:`info`
+  etc. is usually a string, it can in fact be any object. This object is the
+  argument to a :func:`str()` call which is made, in
+  :meth:`LogRecord.getMessage`, to obtain the actual message string. You can
+  use this behavior to pass an instance which may be initialized with a
+  logging message, which redefines :meth:__str__ to return a modified version
+  of that message with the contextual information added.
+
+* Use a specialized :class:`Formatter` subclass to add additional information
+  to the formatted output. The subclass could, for instance, merge some thread
+  local contextual information (or contextual information obtained in some
+  other way) with the output generated by the base :class:`Formatter`.
+
+In each of these three approaches, thread locals can sometimes be a useful way
+of passing contextual information without undue coupling between different
+parts of your code.
+
 .. _network-logging:
 
 Sending and receiving logging events across a network