mirror of https://github.com/python/cpython
423 lines
15 KiB
ReStructuredText
423 lines
15 KiB
ReStructuredText
.. _bltin-exceptions:
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Built-in Exceptions
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===================
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.. index::
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statement: try
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statement: except
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In Python, all exceptions must be instances of a class that derives from
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:class:`BaseException`. In a :keyword:`try` statement with an :keyword:`except`
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clause that mentions a particular class, that clause also handles any exception
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classes derived from that class (but not exception classes from which *it* is
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derived). Two exception classes that are not related via subclassing are never
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equivalent, even if they have the same name.
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.. index:: statement: raise
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The built-in exceptions listed below can be generated by the interpreter or
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built-in functions. Except where mentioned, they have an "associated value"
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indicating the detailed cause of the error. This may be a string or a tuple
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containing several items of information (e.g., an error code and a string
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explaining the code). The associated value is usually passed to the exception
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class's constructor. If the exception class is derived from the standard root
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class :exc:`BaseException`, the associated value is present as the exception
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instance's :attr:`args` attribute.
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User code can raise built-in exceptions. This can be used to test an exception
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handler or to report an error condition "just like" the situation in which the
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interpreter raises the same exception; but beware that there is nothing to
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prevent user code from raising an inappropriate error.
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The built-in exception classes can be sub-classed to define new exceptions;
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programmers are encouraged to at least derive new exceptions from the
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:exc:`Exception` class and not :exc:`BaseException`. More information on
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defining exceptions is available in the Python Tutorial under
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:ref:`tut-userexceptions`.
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The following exceptions are used mostly as base classes for other exceptions.
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.. XXX document with_traceback()
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.. exception:: BaseException
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The base class for all built-in exceptions. It is not meant to be directly
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inherited by user-defined classes (for that use :exc:`Exception`). If
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:func:`bytes` or :func:`str` is called on an instance of this class, the
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representation of the argument(s) to the instance are returned or the empty
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string when there were no arguments. All arguments are stored in :attr:`args`
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as a tuple.
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.. exception:: Exception
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All built-in, non-system-exiting exceptions are derived from this class. All
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user-defined exceptions should also be derived from this class.
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.. exception:: ArithmeticError
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The base class for those built-in exceptions that are raised for various
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arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,
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:exc:`FloatingPointError`.
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.. exception:: LookupError
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The base class for the exceptions that are raised when a key or index used on
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a mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`. This
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can be raised directly by :func:`codecs.lookup`.
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.. exception:: EnvironmentError
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The base class for exceptions that can occur outside the Python system:
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:exc:`IOError`, :exc:`OSError`. When exceptions of this type are created with a
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2-tuple, the first item is available on the instance's :attr:`errno` attribute
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(it is assumed to be an error number), and the second item is available on the
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:attr:`strerror` attribute (it is usually the associated error message). The
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tuple itself is also available on the :attr:`args` attribute.
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When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the
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first two items are available as above, while the third item is available on the
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:attr:`filename` attribute. However, for backwards compatibility, the
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:attr:`args` attribute contains only a 2-tuple of the first two constructor
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arguments.
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The :attr:`filename` attribute is ``None`` when this exception is created with
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other than 3 arguments. The :attr:`errno` and :attr:`strerror` attributes are
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also ``None`` when the instance was created with other than 2 or 3 arguments.
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In this last case, :attr:`args` contains the verbatim constructor arguments as a
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tuple.
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The following exceptions are the exceptions that are usually raised.
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.. exception:: AssertionError
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.. index:: statement: assert
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Raised when an :keyword:`assert` statement fails.
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.. exception:: AttributeError
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Raised when an attribute reference (see :ref:`attribute-references`) or
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assignment fails. (When an object does not support attribute references or
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attribute assignments at all, :exc:`TypeError` is raised.)
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.. exception:: EOFError
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Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)
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hits an end-of-file condition (EOF) without reading any data. (N.B.: the
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:meth:`file.read` and :meth:`file.readline` methods return an empty string
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when they hit EOF.)
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.. exception:: FloatingPointError
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Raised when a floating point operation fails. This exception is always defined,
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but can only be raised when Python is configured with the
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:option:`--with-fpectl` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
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defined in the :file:`pyconfig.h` file.
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.. exception:: GeneratorExit
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Raise when a :term:`generator`\'s :meth:`close` method is called. It
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directly inherits from :exc:`BaseException` instead of :exc:`Exception` since
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it is technically not an error.
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.. exception:: IOError
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Raised when an I/O operation (such as the built-in :func:`print` or
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:func:`open` functions or a method of a :term:`file object`) fails for an
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I/O-related reason, e.g., "file not found" or "disk full".
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This class is derived from :exc:`EnvironmentError`. See the discussion above
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for more information on exception instance attributes.
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.. exception:: ImportError
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Raised when an :keyword:`import` statement fails to find the module definition
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or when a ``from ... import`` fails to find a name that is to be imported.
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.. exception:: IndexError
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Raised when a sequence subscript is out of range. (Slice indices are
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silently truncated to fall in the allowed range; if an index is not an
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integer, :exc:`TypeError` is raised.)
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.. XXX xref to sequences
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.. exception:: KeyError
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Raised when a mapping (dictionary) key is not found in the set of existing keys.
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.. XXX xref to mapping objects?
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.. exception:: KeyboardInterrupt
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Raised when the user hits the interrupt key (normally :kbd:`Control-C` or
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:kbd:`Delete`). During execution, a check for interrupts is made
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regularly. The exception inherits from :exc:`BaseException` so as to not be
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accidentally caught by code that catches :exc:`Exception` and thus prevent
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the interpreter from exiting.
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.. exception:: MemoryError
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Raised when an operation runs out of memory but the situation may still be
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rescued (by deleting some objects). The associated value is a string indicating
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what kind of (internal) operation ran out of memory. Note that because of the
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underlying memory management architecture (C's :cfunc:`malloc` function), the
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interpreter may not always be able to completely recover from this situation; it
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nevertheless raises an exception so that a stack traceback can be printed, in
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case a run-away program was the cause.
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.. exception:: NameError
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Raised when a local or global name is not found. This applies only to
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unqualified names. The associated value is an error message that includes the
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name that could not be found.
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.. exception:: NotImplementedError
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This exception is derived from :exc:`RuntimeError`. In user defined base
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classes, abstract methods should raise this exception when they require derived
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classes to override the method.
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.. exception:: OSError
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.. index:: module: errno
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This exception is derived from :exc:`EnvironmentError`. It is raised when a
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function returns a system-related error (not for illegal argument types or
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other incidental errors). The :attr:`errno` attribute is a numeric error
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code from :cdata:`errno`, and the :attr:`strerror` attribute is the
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corresponding string, as would be printed by the C function :cfunc:`perror`.
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See the module :mod:`errno`, which contains names for the error codes defined
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by the underlying operating system.
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For exceptions that involve a file system path (such as :func:`chdir` or
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:func:`unlink`), the exception instance will contain a third attribute,
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:attr:`filename`, which is the file name passed to the function.
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.. exception:: OverflowError
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Raised when the result of an arithmetic operation is too large to be
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represented. This cannot occur for integers (which would rather raise
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:exc:`MemoryError` than give up). Because of the lack of standardization of
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floating point exception handling in C, most floating point operations also
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aren't checked.
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.. exception:: ReferenceError
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This exception is raised when a weak reference proxy, created by the
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:func:`weakref.proxy` function, is used to access an attribute of the referent
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after it has been garbage collected. For more information on weak references,
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see the :mod:`weakref` module.
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.. exception:: RuntimeError
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Raised when an error is detected that doesn't fall in any of the other
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categories. The associated value is a string indicating what precisely went
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wrong. (This exception is mostly a relic from a previous version of the
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interpreter; it is not used very much any more.)
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.. exception:: StopIteration
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Raised by built-in function :func:`next` and an :term:`iterator`\'s
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:meth:`__next__` method to signal that there are no further values.
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.. exception:: SyntaxError
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Raised when the parser encounters a syntax error. This may occur in an
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:keyword:`import` statement, in a call to the built-in functions :func:`exec`
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or :func:`eval`, or when reading the initial script or standard input
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(also interactively).
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Instances of this class have attributes :attr:`filename`, :attr:`lineno`,
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:attr:`offset` and :attr:`text` for easier access to the details. :func:`str`
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of the exception instance returns only the message.
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.. exception:: SystemError
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Raised when the interpreter finds an internal error, but the situation does not
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look so serious to cause it to abandon all hope. The associated value is a
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string indicating what went wrong (in low-level terms).
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You should report this to the author or maintainer of your Python interpreter.
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Be sure to report the version of the Python interpreter (``sys.version``; it is
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also printed at the start of an interactive Python session), the exact error
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message (the exception's associated value) and if possible the source of the
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program that triggered the error.
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.. exception:: SystemExit
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This exception is raised by the :func:`sys.exit` function. When it is not
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handled, the Python interpreter exits; no stack traceback is printed. If the
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associated value is an integer, it specifies the system exit status (passed
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to C's :cfunc:`exit` function); if it is ``None``, the exit status is zero;
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if it has another type (such as a string), the object's value is printed and
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the exit status is one.
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Instances have an attribute :attr:`code` which is set to the proposed exit
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status or error message (defaulting to ``None``). Also, this exception derives
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directly from :exc:`BaseException` and not :exc:`Exception`, since it is not
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technically an error.
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A call to :func:`sys.exit` is translated into an exception so that clean-up
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handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be
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executed, and so that a debugger can execute a script without running the risk
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of losing control. The :func:`os._exit` function can be used if it is
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absolutely positively necessary to exit immediately (for example, in the child
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process after a call to :func:`fork`).
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The exception inherits from :exc:`BaseException` instead of :exc:`Exception` so
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that it is not accidentally caught by code that catches :exc:`Exception`. This
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allows the exception to properly propagate up and cause the interpreter to exit.
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.. exception:: TypeError
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Raised when an operation or function is applied to an object of inappropriate
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type. The associated value is a string giving details about the type mismatch.
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.. exception:: UnboundLocalError
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Raised when a reference is made to a local variable in a function or method, but
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no value has been bound to that variable. This is a subclass of
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:exc:`NameError`.
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.. exception:: UnicodeError
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Raised when a Unicode-related encoding or decoding error occurs. It is a
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subclass of :exc:`ValueError`.
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.. exception:: UnicodeEncodeError
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Raised when a Unicode-related error occurs during encoding. It is a subclass of
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:exc:`UnicodeError`.
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.. exception:: UnicodeDecodeError
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Raised when a Unicode-related error occurs during decoding. It is a subclass of
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:exc:`UnicodeError`.
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.. exception:: UnicodeTranslateError
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Raised when a Unicode-related error occurs during translating. It is a subclass
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of :exc:`UnicodeError`.
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.. exception:: ValueError
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Raised when a built-in operation or function receives an argument that has the
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right type but an inappropriate value, and the situation is not described by a
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more precise exception such as :exc:`IndexError`.
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.. exception:: VMSError
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Only available on VMS. Raised when a VMS-specific error occurs.
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.. exception:: WindowsError
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Raised when a Windows-specific error occurs or when the error number does not
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correspond to an :cdata:`errno` value. The :attr:`winerror` and
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:attr:`strerror` values are created from the return values of the
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:cfunc:`GetLastError` and :cfunc:`FormatMessage` functions from the Windows
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Platform API. The :attr:`errno` value maps the :attr:`winerror` value to
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corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.
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.. exception:: ZeroDivisionError
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Raised when the second argument of a division or modulo operation is zero. The
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associated value is a string indicating the type of the operands and the
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operation.
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The following exceptions are used as warning categories; see the :mod:`warnings`
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module for more information.
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.. exception:: Warning
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Base class for warning categories.
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.. exception:: UserWarning
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Base class for warnings generated by user code.
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.. exception:: DeprecationWarning
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Base class for warnings about deprecated features.
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.. exception:: PendingDeprecationWarning
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Base class for warnings about features which will be deprecated in the future.
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.. exception:: SyntaxWarning
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Base class for warnings about dubious syntax
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.. exception:: RuntimeWarning
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Base class for warnings about dubious runtime behavior.
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.. exception:: FutureWarning
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Base class for warnings about constructs that will change semantically in the
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future.
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.. exception:: ImportWarning
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Base class for warnings about probable mistakes in module imports.
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.. exception:: UnicodeWarning
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Base class for warnings related to Unicode.
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.. exception:: BytesWarning
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Base class for warnings related to :class:`bytes` and :class:`buffer`.
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Exception hierarchy
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-------------------
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The class hierarchy for built-in exceptions is:
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.. literalinclude:: ../../Lib/test/exception_hierarchy.txt
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