.. _bltin-exceptions:

Built-in Exceptions

===================

.. module:: exceptions

   :synopsis: Standard exception classes.

Exceptions should be class objects.   The exceptions are defined in the module

:mod:`exceptions`.  This module never needs to be imported explicitly: the

exceptions are provided in the built-in namespace as well as the

:mod:`exceptions` module.

.. index::

   statement: try

   statement: except

For class exceptions, in a :keyword:`try` statement with an :keyword:`except`

clause that mentions a particular class, that clause also handles any exception

classes derived from that class (but not exception classes from which *it* is

derived).  Two exception classes that are not related via subclassing are never

equivalent, even if they have the same name.

.. index:: statement: raise

The built-in exceptions listed below can be generated by the interpreter or

built-in functions.  Except where mentioned, they have an "associated value"

indicating the detailed cause of the error. This may be a string or a tuple

containing several items of information (e.g., an error code and a string

explaining the code). The associated value is the second argument to the

:keyword:`raise` statement.  If the exception class is derived from the standard

root class :exc:`BaseException`, the associated value is present as the

exception instance's :attr:`args` attribute.

User code can raise built-in exceptions.  This can be used to test an exception

handler or to report an error condition "just like" the situation in which the

interpreter raises the same exception; but beware that there is nothing to

prevent user code from raising an inappropriate error.

The built-in exception classes can be sub-classed to define new exceptions;

programmers are encouraged to at least derive new exceptions from the

:exc:`Exception` class and not :exc:`BaseException`.  More information on

defining exceptions is available in the Python Tutorial under

:ref:`tut-userexceptions`.

The following exceptions are only used as base classes for other exceptions.

.. exception:: BaseException

   The base class for all built-in exceptions.  It is not meant to be directly

   inherited by user-defined classes (for that use :exc:`Exception`).  If

   :func:`str` or :func:`unicode` is called on an instance of this class, the

   representation of the argument(s) to the instance are returned or the emptry

   string when there were no arguments.  All arguments are  stored in :attr:`args`

   as a tuple.

   .. versionadded:: 2.5

.. exception:: Exception

   All built-in, non-system-exiting exceptions are derived from this class.  All

   user-defined exceptions should also be derived from this class.

   .. versionchanged:: 2.5

      Changed to inherit from :exc:`BaseException`.

.. exception:: StandardError

   The base class for all built-in exceptions except :exc:`StopIteration`,

   :exc:`GeneratorExit`, :exc:`KeyboardInterrupt` and :exc:`SystemExit`.

   :exc:`StandardError` itself is derived from :exc:`Exception`.

.. exception:: ArithmeticError

   The base class for those built-in exceptions that are raised for various

   arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,

   :exc:`FloatingPointError`.

.. exception:: LookupError

   The base class for the exceptions that are raised when a key or index used on a

   mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`.  This can be

   raised directly by :func:`sys.setdefaultencoding`.

.. exception:: EnvironmentError

   The base class for exceptions that can occur outside the Python system:

   :exc:`IOError`, :exc:`OSError`.  When exceptions of this type are created with a

   2-tuple, the first item is available on the instance's :attr:`errno` attribute

   (it is assumed to be an error number), and the second item is available on the

   :attr:`strerror` attribute (it is usually the associated error message).  The

   tuple itself is also available on the :attr:`args` attribute.

   .. versionadded:: 1.5.2

   When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the

   first two items are available as above, while the third item is available on the

   :attr:`filename` attribute.  However, for backwards compatibility, the

   :attr:`args` attribute contains only a 2-tuple of the first two constructor

   arguments.

   The :attr:`filename` attribute is ``None`` when this exception is created with

   other than 3 arguments.  The :attr:`errno` and :attr:`strerror` attributes are

   also ``None`` when the instance was created with other than 2 or 3 arguments.

   In this last case, :attr:`args` contains the verbatim constructor arguments as a

   tuple.

The following exceptions are the exceptions that are actually raised.

.. exception:: AssertionError

   .. index:: statement: assert

   Raised when an :keyword:`assert` statement fails.

.. exception:: AttributeError

   Raised when an attribute reference (see :ref:`attribute-references`) or

   assignment fails.  (When an object does not support attribute references or

   attribute assignments at all, :exc:`TypeError` is raised.)

.. exception:: EOFError

   Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)

   hits an end-of-file condition (EOF) without reading any data. (N.B.: the

   :meth:`file.read` and :meth:`file.readline` methods return an empty string

   when they hit EOF.)

.. exception:: FloatingPointError

   Raised when a floating point operation fails.  This exception is always defined,

   but can only be raised when Python is configured with the

   :option:`--with-fpectl` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is

   defined in the :file:`pyconfig.h` file.

.. exception:: GeneratorExit

   Raise when a :term:`generator`\'s :meth:`close` method is called.  It

   directly inherits from :exc:`BaseException` instead of :exc:`StandardError` since

   it is technically not an error.

   .. versionadded:: 2.5

   .. versionchanged:: 2.6

      Changed to inherit from :exc:`BaseException`.

.. exception:: IOError

   Raised when an I/O operation (such as a :keyword:`print` statement, the built-in

   :func:`open` function or a method of a file object) fails for an I/O-related

   reason, e.g., "file not found" or "disk full".

   This class is derived from :exc:`EnvironmentError`.  See the discussion above

   for more information on exception instance attributes.

   .. versionchanged:: 2.6

      Changed :exc:`socket.error` to use this as a base class.

.. exception:: ImportError

   Raised when an :keyword:`import` statement fails to find the module definition

   or when a ``from ... import`` fails to find a name that is to be imported.

.. exception:: IndexError

   Raised when a sequence subscript is out of range.  (Slice indices are silently

   truncated to fall in the allowed range; if an index is not a plain integer,

   :exc:`TypeError` is raised.)

   .. XXX xref to sequences

.. exception:: KeyError

   Raised when a mapping (dictionary) key is not found in the set of existing keys.

   .. XXX xref to mapping objects?

.. exception:: KeyboardInterrupt

   Raised when the user hits the interrupt key (normally :kbd:`Control-C` or

   :kbd:`Delete`).  During execution, a check for interrupts is made regularly.

   Interrupts typed when a built-in function :func:`input` or :func:`raw_input` is

   waiting for input also raise this exception. The exception inherits from

   :exc:`BaseException` so as to not be accidentally caught by code that catches

   :exc:`Exception` and thus prevent the interpreter from exiting.

   .. versionchanged:: 2.5

      Changed to inherit from :exc:`BaseException`.

.. exception:: MemoryError

   Raised when an operation runs out of memory but the situation may still be

   rescued (by deleting some objects).  The associated value is a string indicating

   what kind of (internal) operation ran out of memory. Note that because of the

   underlying memory management architecture (C's :cfunc:`malloc` function), the

   interpreter may not always be able to completely recover from this situation; it

   nevertheless raises an exception so that a stack traceback can be printed, in

   case a run-away program was the cause.

.. exception:: NameError

   Raised when a local or global name is not found.  This applies only to

   unqualified names.  The associated value is an error message that includes the

   name that could not be found.

.. exception:: NotImplementedError

   This exception is derived from :exc:`RuntimeError`.  In user defined base

   classes, abstract methods should raise this exception when they require derived

   classes to override the method.

   .. versionadded:: 1.5.2

.. exception:: OSError

   .. index:: module: errno

   This exception is derived from :exc:`EnvironmentError`.  It is raised when a

   function returns a system-related error (not for illegal argument types or

   other incidental errors).  The :attr:`errno` attribute is a numeric error

   code from :cdata:`errno`, and the :attr:`strerror` attribute is the

   corresponding string, as would be printed by the C function :cfunc:`perror`.

   See the module :mod:`errno`, which contains names for the error codes defined

   by the underlying operating system.

   For exceptions that involve a file system path (such as :func:`chdir` or

   :func:`unlink`), the exception instance will contain a third attribute,

   :attr:`filename`, which is the file name passed to the function.

   .. versionadded:: 1.5.2

.. exception:: OverflowError

   Raised when the result of an arithmetic operation is too large to be

   represented.  This cannot occur for long integers (which would rather raise

   :exc:`MemoryError` than give up) and for most operations with plain integers,

   which return a long integer instead.  Because of the lack of standardization

   of floating point exception handling in C, most floating point operations

   also aren't checked.

.. exception:: ReferenceError

   This exception is raised when a weak reference proxy, created by the

   :func:`weakref.proxy` function, is used to access an attribute of the referent

   after it has been garbage collected. For more information on weak references,

   see the :mod:`weakref` module.

   .. versionadded:: 2.2

      Previously known as the :exc:`weakref.ReferenceError` exception.

.. exception:: RuntimeError

   Raised when an error is detected that doesn't fall in any of the other

   categories.  The associated value is a string indicating what precisely went

   wrong.  (This exception is mostly a relic from a previous version of the

   interpreter; it is not used very much any more.)

.. exception:: StopIteration

   Raised by an :term:`iterator`\'s :meth:`next` method to signal that there are

   no further values.  This is derived from :exc:`Exception` rather than

   :exc:`StandardError`, since this is not considered an error in its normal

   application.

   .. versionadded:: 2.2

.. exception:: SyntaxError

   Raised when the parser encounters a syntax error.  This may occur in an

   :keyword:`import` statement, in an :keyword:`exec` statement, in a call to the

   built-in function :func:`eval` or :func:`input`, or when reading the initial

   script or standard input (also interactively).

   Instances of this class have attributes :attr:`filename`, :attr:`lineno`,

   :attr:`offset` and :attr:`text` for easier access to the details.  :func:`str`

   of the exception instance returns only the message.

.. exception:: SystemError

   Raised when the interpreter finds an internal error, but the situation does not

   look so serious to cause it to abandon all hope. The associated value is a

   string indicating what went wrong (in low-level terms).

   You should report this to the author or maintainer of your Python interpreter.

   Be sure to report the version of the Python interpreter (``sys.version``; it is

   also printed at the start of an interactive Python session), the exact error

   message (the exception's associated value) and if possible the source of the

   program that triggered the error.

.. exception:: SystemExit

   This exception is raised by the :func:`sys.exit` function.  When it is not

   handled, the Python interpreter exits; no stack traceback is printed.  If the

   associated value is a plain integer, it specifies the system exit status (passed

   to C's :cfunc:`exit` function); if it is ``None``, the exit status is zero; if

   it has another type (such as a string), the object's value is printed and the

   exit status is one.

   Instances have an attribute :attr:`code` which is set to the proposed exit

   status or error message (defaulting to ``None``). Also, this exception derives

   directly from :exc:`BaseException` and not :exc:`StandardError`, since it is not

   technically an error.

   A call to :func:`sys.exit` is translated into an exception so that clean-up

   handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be

   executed, and so that a debugger can execute a script without running the risk

   of losing control.  The :func:`os._exit` function can be used if it is

   absolutely positively necessary to exit immediately (for example, in the child

   process after a call to :func:`fork`).

   The exception inherits from :exc:`BaseException` instead of :exc:`StandardError`

   or :exc:`Exception` so that it is not accidentally caught by code that catches

   :exc:`Exception`.  This allows the exception to properly propagate up and cause

   the interpreter to exit.

   .. versionchanged:: 2.5

      Changed to inherit from :exc:`BaseException`.

.. exception:: TypeError

   Raised when an operation or function is applied to an object of inappropriate

   type.  The associated value is a string giving details about the type mismatch.

.. exception:: UnboundLocalError

   Raised when a reference is made to a local variable in a function or method, but

   no value has been bound to that variable.  This is a subclass of

   :exc:`NameError`.

   .. versionadded:: 2.0

.. exception:: UnicodeError

   Raised when a Unicode-related encoding or decoding error occurs.  It is a

   subclass of :exc:`ValueError`.

   .. versionadded:: 2.0

.. exception:: UnicodeEncodeError

   Raised when a Unicode-related error occurs during encoding.  It is a subclass of

   :exc:`UnicodeError`.

   .. versionadded:: 2.3

.. exception:: UnicodeDecodeError

   Raised when a Unicode-related error occurs during decoding.  It is a subclass of

   :exc:`UnicodeError`.

   .. versionadded:: 2.3

.. exception:: UnicodeTranslateError

   Raised when a Unicode-related error occurs during translating.  It is a subclass

   of :exc:`UnicodeError`.

   .. versionadded:: 2.3

.. exception:: ValueError

   Raised when a built-in operation or function receives an argument that has the

   right type but an inappropriate value, and the situation is not described by a

   more precise exception such as :exc:`IndexError`.

.. exception:: WindowsError

   Raised when a Windows-specific error occurs or when the error number does not

   correspond to an :cdata:`errno` value.  The :attr:`winerror` and

   :attr:`strerror` values are created from the return values of the

   :cfunc:`GetLastError` and :cfunc:`FormatMessage` functions from the Windows

   Platform API. The :attr:`errno` value maps the :attr:`winerror` value to

   corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.

   .. versionadded:: 2.0

   .. versionchanged:: 2.5

      Previous versions put the :cfunc:`GetLastError` codes into :attr:`errno`.

.. exception:: ZeroDivisionError

   Raised when the second argument of a division or modulo operation is zero.  The

   associated value is a string indicating the type of the operands and the

   operation.

The following exceptions are used as warning categories; see the :mod:`warnings`

module for more information.

.. exception:: Warning

   Base class for warning categories.

.. exception:: UserWarning

   Base class for warnings generated by user code.

.. exception:: DeprecationWarning

   Base class for warnings about deprecated features.

.. exception:: PendingDeprecationWarning

   Base class for warnings about features which will be deprecated in the future.

.. exception:: SyntaxWarning

   Base class for warnings about dubious syntax

.. exception:: RuntimeWarning

   Base class for warnings about dubious runtime behavior.

.. exception:: FutureWarning

   Base class for warnings about constructs that will change semantically in the

   future.

.. exception:: ImportWarning

   Base class for warnings about probable mistakes in module imports.

   .. versionadded:: 2.5

.. exception:: UnicodeWarning

   Base class for warnings related to Unicode.

   .. versionadded:: 2.5

The class hierarchy for built-in exceptions is:

.. literalinclude:: ../../Lib/test/exception_hierarchy.txt