
Built-in Exceptions
*******************

Exceptions should be class objects.   The exceptions are defined in
the module ``exceptions``.  This module never needs to be imported
explicitly: the exceptions are provided in the built-in namespace as
well as the ``exceptions`` module.

For class exceptions, in a ``try`` statement with an ``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.

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 ``raise`` statement.  If the
exception class is derived from the standard root class
``BaseException``, the associated value is present as the exception
instance's ``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 ``Exception`` class and not ``BaseException``.
More information on defining exceptions is available in the Python
Tutorial under *User-defined Exceptions*.

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

exception 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
   ``Exception``).  If ``str()`` or ``unicode()`` is called on an
   instance of this class, the representation of the argument(s) to
   the instance are returned or the empty string when there were no
   arguments.  All arguments are  stored in ``args`` as a tuple.

exception 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.

exception exception ArithmeticError

   The base class for those built-in exceptions that are raised for
   various arithmetic errors: ``OverflowError``,
   ``ZeroDivisionError``, ``FloatingPointError``.

exception exception LookupError

   The base class for the exceptions that are raised when a key or
   index used on a mapping or sequence is invalid: ``IndexError``,
   ``KeyError``.  This can be raised directly by ``codecs.lookup()``.

exception exception EnvironmentError

   The base class for exceptions that can occur outside the Python
   system: ``IOError``, ``OSError``.  When exceptions of this type are
   created with a 2-tuple, the first item is available on the
   instance's ``errno`` attribute (it is assumed to be an error
   number), and the second item is available on the ``strerror``
   attribute (it is usually the associated error message).  The tuple
   itself is also available on the ``args`` attribute.

   When an ``EnvironmentError`` exception is instantiated with a
   3-tuple, the first two items are available as above, while the
   third item is available on the ``filename`` attribute.  However,
   for backwards compatibility, the ``args`` attribute contains only a
   2-tuple of the first two constructor arguments.

   The ``filename`` attribute is ``None`` when this exception is
   created with other than 3 arguments.  The ``errno`` and
   ``strerror`` attributes are also ``None`` when the instance was
   created with other than 2 or 3 arguments. In this last case,
   ``args`` contains the verbatim constructor arguments as a tuple.

The following exceptions are the exceptions that are actually raised.

exception exception AssertionError

   Raised when an ``assert`` statement fails.

exception exception AttributeError

   Raised when an attribute reference (see *Attribute references*) or
   assignment fails.  (When an object does not support attribute
   references or attribute assignments at all, ``TypeError`` is
   raised.)

exception exception EOFError

   Raised when one of the built-in functions (``input()`` or
   ``raw_input()``) hits an end-of-file condition (EOF) without
   reading any data. (N.B.: the ``file.read()`` and
   ``file.readline()`` methods return an empty string when they hit
   EOF.)

exception 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 *--with-fpectl* option, or the ``WANT_SIGFPE_HANDLER``
   symbol is defined in the ``pyconfig.h`` file.

exception exception GeneratorExit

   Raise when a *generator*'s ``close()`` method is called.  It
   directly inherits from ``BaseException`` instead of ``Exception``
   since it is technically not an error.

exception exception IOError

   Raised when an I/O operation (such as the built-in ``print()`` or
   ``open()`` functions 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 ``EnvironmentError``.  See the
   discussion above for more information on exception instance
   attributes.

exception exception ImportError

   Raised when an ``import`` statement fails to find the module
   definition or when a ``from ... import`` fails to find a name that
   is to be imported.

exception 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 an integer, ``TypeError`` is raised.)

exception exception KeyError

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

exception exception KeyboardInterrupt

   Raised when the user hits the interrupt key (normally ``Control-C``
   or ``Delete``).  During execution, a check for interrupts is made
   regularly. The exception inherits from ``BaseException`` so as to
   not be accidentally caught by code that catches ``Exception`` and
   thus prevent the interpreter from exiting.

exception 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 ``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 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 exception NotImplementedError

   This exception is derived from ``RuntimeError``.  In user defined
   base classes, abstract methods should raise this exception when
   they require derived classes to override the method.

exception exception OSError

   This exception is derived from ``EnvironmentError``.  It is raised
   when a function returns a system-related error (not for illegal
   argument types or other incidental errors).  The ``errno``
   attribute is a numeric error code from ``errno``, and the
   ``strerror`` attribute is the corresponding string, as would be
   printed by the C function ``perror()``. See the module ``errno``,
   which contains names for the error codes defined by the underlying
   operating system.

   For exceptions that involve a file system path (such as ``chdir()``
   or ``unlink()``), the exception instance will contain a third
   attribute, ``filename``, which is the file name passed to the
   function.

exception exception OverflowError

   Raised when the result of an arithmetic operation is too large to
   be represented.  This cannot occur for integers (which would rather
   raise ``MemoryError`` than give up).  Because of the lack of
   standardization of floating point exception handling in C, most
   floating point operations also aren't checked.

exception exception ReferenceError

   This exception is raised when a weak reference proxy, created by
   the ``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 ``weakref`` module.

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 exception StopIteration

   Raised by builtin ``next()`` and an *iterator*'s ``__next__()``
   method to signal that there are no further values.

exception exception SyntaxError

   Raised when the parser encounters a syntax error.  This may occur
   in an ``import`` statement, in a call to the built-in functions
   ``exec()`` or ``eval()``, or when reading the initial script or
   standard input (also interactively).

   Instances of this class have attributes ``filename``, ``lineno``,
   ``offset`` and ``text`` for easier access to the details.
   ``str()`` of the exception instance returns only the message.

exception 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 exception SystemExit

   This exception is raised by the ``sys.exit()`` function.  When it
   is not handled, the Python interpreter exits; no stack traceback is
   printed.  If the associated value is an integer, it specifies the
   system exit status (passed to C's ``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 ``code`` which is set to the proposed
   exit status or error message (defaulting to ``None``). Also, this
   exception derives directly from ``BaseException`` and not
   ``Exception``, since it is not technically an error.

   A call to ``sys.exit()`` is translated into an exception so that
   clean-up handlers (``finally`` clauses of ``try`` statements) can
   be executed, and so that a debugger can execute a script without
   running the risk of losing control.  The ``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
   ``fork()``).

   The exception inherits from ``BaseException`` instead of
   ``Exception`` so that it is not accidentally caught by code that
   catches ``Exception``.  This allows the exception to properly
   propagate up and cause the interpreter to exit.

exception 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 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 ``NameError``.

exception exception UnicodeError

   Raised when a Unicode-related encoding or decoding error occurs.
   It is a subclass of ``ValueError``.

exception exception UnicodeEncodeError

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

exception exception UnicodeDecodeError

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

exception exception UnicodeTranslateError

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

exception 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
   ``IndexError``.

exception exception VMSError

   Only available on VMS.  Raised when a VMS-specific error occurs.

exception exception WindowsError

   Raised when a Windows-specific error occurs or when the error
   number does not correspond to an ``errno`` value.  The ``winerror``
   and ``strerror`` values are created from the return values of the
   ``GetLastError()`` and ``FormatMessage()`` functions from the
   Windows Platform API. The ``errno`` value maps the ``winerror``
   value to corresponding ``errno.h`` values. This is a subclass of
   ``OSError``.

exception 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
``warnings`` module for more information.

exception exception Warning

   Base class for warning categories.

exception exception UserWarning

   Base class for warnings generated by user code.

exception exception DeprecationWarning

   Base class for warnings about deprecated features.

exception exception PendingDeprecationWarning

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

exception exception SyntaxWarning

   Base class for warnings about dubious syntax

exception exception RuntimeWarning

   Base class for warnings about dubious runtime behavior.

exception exception FutureWarning

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

exception exception ImportWarning

   Base class for warnings about probable mistakes in module imports.

exception exception UnicodeWarning

   Base class for warnings related to Unicode.

exception exception BytesWarning

   Base class for warnings related to ``bytes`` and ``buffer``.


Exception hierarchy
===================

The class hierarchy for built-in exceptions is:

   BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
         +-- StopIteration
         +-- ArithmeticError
         |    +-- FloatingPointError
         |    +-- OverflowError
         |    +-- ZeroDivisionError
         +-- AssertionError
         +-- AttributeError
         +-- BufferError
         +-- EnvironmentError
         |    +-- IOError
         |    +-- OSError
         |         +-- WindowsError (Windows)
         |         +-- VMSError (VMS)
         +-- EOFError
         +-- ImportError
         +-- LookupError
         |    +-- IndexError
         |    +-- KeyError
         +-- MemoryError
         +-- NameError
         |    +-- UnboundLocalError
         +-- ReferenceError
         +-- RuntimeError
         |    +-- NotImplementedError
         +-- SyntaxError
         |    +-- IndentationError
         |         +-- TabError
         +-- SystemError
         +-- TypeError
         +-- ValueError
         |    +-- UnicodeError
         |         +-- UnicodeDecodeError
         |         +-- UnicodeEncodeError
         |         +-- UnicodeTranslateError
         +-- Warning
              +-- DeprecationWarning
              +-- PendingDeprecationWarning
              +-- RuntimeWarning
              +-- SyntaxWarning
              +-- UserWarning
              +-- FutureWarning
              +-- ImportWarning
              +-- UnicodeWarning
              +-- BytesWarning
