.. _bltin-exceptions:

Built-in Exceptions
===================

.. index::
   statement: try
   statement: except

In Python, all exceptions must be instances of a class that derives from
:class:`BaseException`.  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 used mostly as base classes for other exceptions.

.. XXX document with_traceback()

.. 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:`bytes` or :func:`str` 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 :attr:`args`
   as a tuple.


.. 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:: 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:`codecs.lookup`.


.. 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.

   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 usually 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:`Exception` since
   it is technically not an error.


.. exception:: IOError

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


.. 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 an
   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. 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.


.. 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.


.. 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.


.. 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
   :exc:`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:: 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.


.. 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 built-in function :func:`next` and an :term:`iterator`\'s
   :meth:`__next__` method to signal that there are no further values.


.. exception:: SyntaxError

   Raised when the parser encounters a syntax error.  This may occur in an
   :keyword:`import` statement, in a call to the built-in functions :func:`exec`
   or :func:`eval`, 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 an 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:`Exception`, 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:`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.


.. 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`.


.. exception:: UnicodeError

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


.. exception:: UnicodeEncodeError

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


.. exception:: UnicodeDecodeError

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


.. exception:: UnicodeTranslateError

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


.. 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:: VMSError

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


.. 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`.


.. 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.


.. exception:: UnicodeWarning

   Base class for warnings related to Unicode.

.. exception:: BytesWarning

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

Exception hierarchy
-------------------

The class hierarchy for built-in exceptions is:

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