exceptions.rst

.. _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 of
several items of information (e.g., an error code and a string explaining the
code).  The associated value is usually passed as arguments to the exception
class's constructor.

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 subclassed to define new exceptions;
programmers are encouraged to derive new exceptions from the :exc:`Exception`
class or one of its subclasses, and not from :exc:`BaseException`.  More
information on defining exceptions is available in the Python Tutorial under
:ref:`tut-userexceptions`.

When raising (or re-raising) an exception in an :keyword:`except` or
:keyword:`finally` clause
:attr:`__context__` is automatically set to the last exception caught; if the
new exception is not handled the traceback that is eventually displayed will
include the originating exception(s) and the final exception.

When raising a new exception (rather than using a bare ``raise`` to re-raise
the exception currently being handled), the implicit exception context can be
supplemented with an explicit cause by using :keyword:`from` with
:keyword:`raise`::

   raise new_exc from original_exc

The expression following :keyword:`from` must be an exception or ``None``. It
will be set as :attr:`__cause__` on the raised exception. Setting
:attr:`__cause__` also implicitly sets the :attr:`__suppress_context__`
attribute to ``True``, so that using ``raise new_exc from None``
effectively replaces the old exception with the new one for display
purposes (e.g. converting :exc:`KeyError` to :exc:`AttributeError`, while
leaving the old exception available in :attr:`__context__` for introspection
when debugging.

The default traceback display code shows these chained exceptions in
addition to the traceback for the exception itself. An explicitly chained
exception in :attr:`__cause__` is always shown when present. An implicitly
chained exception in :attr:`__context__` is shown only if :attr:`__cause__`
is :const:`None` and :attr:`__suppress_context__` is false.

In either case, the exception itself is always shown after any chained
exceptions so that the final line of the traceback always shows the last
exception that was raised.


Base classes
------------

The following exceptions are used mostly 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` 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.

   .. attribute:: args

      The tuple of arguments given to the exception constructor.  Some built-in
      exceptions (like :exc:`OSError`) expect a certain number of arguments and
      assign a special meaning to the elements of this tuple, while others are
      usually called only with a single string giving an error message.

   .. method:: with_traceback(tb)

      This method sets *tb* as the new traceback for the exception and returns
      the exception object.  It is usually used in exception handling code like
      this::

         try:
             ...
         except SomeException:
             tb = sys.exc_info()[2]
             raise OtherException(...).with_traceback(tb)


.. 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:: BufferError

   Raised when a :ref:`buffer <bufferobjects>` related operation cannot be
   performed.


.. 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`.


Concrete exceptions
-------------------

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 the :func:`input` function hits an end-of-file condition (EOF)
   without reading any data. (N.B.: the :meth:`io.IOBase.read` and
   :meth:`io.IOBase.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
   ``--with-fpectl`` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
   defined in the :file:`pyconfig.h` file.


.. exception:: GeneratorExit

   Raised when a :term:`generator` or :term:`coroutine` is closed;
   see :meth:`generator.close` and :meth:`coroutine.close`.  It
   directly inherits from :exc:`BaseException` instead of :exc:`Exception` since
   it is technically not an error.


.. 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.

   The :attr:`name` and :attr:`path` attributes can be set using keyword-only
   arguments to the constructor. When set they represent the name of the module
   that was attempted to be imported and the path to any file which triggered
   the exception, respectively.

   .. versionchanged:: 3.3
      Added the :attr:`name` and :attr:`path` attributes.


.. 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 :c:func:`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([arg])
               OSError(errno, strerror[, filename[, winerror[, filename2]]])

   .. index:: module: errno

   This exception is raised when a system function returns a system-related
   error, including I/O failures such as "file not found" or "disk full"
   (not for illegal argument types or other incidental errors).

   The second form of the constructor sets the corresponding attributes,
   described below.  The attributes default to :const:`None` if not
   specified.  For backwards compatibility, if three arguments are passed,
   the :attr:`~BaseException.args` attribute contains only a 2-tuple
   of the first two constructor arguments.

   The constructor often actually returns a subclass of :exc:`OSError`, as
   described in `OS exceptions`_ below.  The particular subclass depends on
   the final :attr:`.errno` value.  This behaviour only occurs when
   constructing :exc:`OSError` directly or via an alias, and is not
   inherited when subclassing.

   .. attribute:: errno

      A numeric error code from the C variable :c:data:`errno`.

   .. attribute:: winerror

      Under Windows, this gives you the native
      Windows error code.  The :attr:`.errno` attribute is then an approximate
      translation, in POSIX terms, of that native error code.

      Under Windows, if the *winerror* constructor argument is an integer,
      the :attr:`.errno` attribute is determined from the Windows error code,
      and the *errno* argument is ignored.  On other platforms, the
      *winerror* argument is ignored, and the :attr:`winerror` attribute
      does not exist.

   .. attribute:: strerror

      The corresponding error message, as provided by
      the operating system.  It is formatted by the C
      functions :c:func:`perror` under POSIX, and :c:func:`FormatMessage`
      under Windows.

   .. attribute:: filename
                  filename2

      For exceptions that involve a file system path (such as :func:`open` or
      :func:`os.unlink`), :attr:`filename` is the file name passed to the function.
      For functions that involve two file system paths (such as
      :func:`os.rename`), :attr:`filename2` corresponds to the second
      file name passed to the function.


   .. versionchanged:: 3.3
      :exc:`EnvironmentError`, :exc:`IOError`, :exc:`WindowsError`,
      :exc:`socket.error`, :exc:`select.error` and
      :exc:`mmap.error` have been merged into :exc:`OSError`, and the
      constructor may return a subclass.

   .. versionchanged:: 3.4
      The :attr:`filename` attribute is now the original file name passed to
      the function, instead of the name encoded to or decoded from the
      filesystem encoding.  Also, the *filename2* constructor argument and
      attribute was added.


.. 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).  However, for historical reasons,
   OverflowError is sometimes raised for integers that are outside a required
   range.   Because of the lack of standardization of floating point exception
   handling in C, most floating point operations are not checked.


.. exception:: RecursionError

   This exception is derived from :exc:`RuntimeError`.  It is raised when the
   interpreter detects that the maximum recursion depth (see
   :func:`sys.getrecursionlimit`) is exceeded.

   .. versionadded:: 3.5
      Previously, a plain :exc:`RuntimeError` was raised.


.. 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.


.. exception:: StopIteration

   Raised by built-in function :func:`next` and an :term:`iterator`\'s
   :meth:`~iterator.__next__` method to signal that there are no further
   items produced by the iterator.

   The exception object has a single attribute :attr:`value`, which is
   given as an argument when constructing the exception, and defaults
   to :const:`None`.

   When a :term:`generator` or :term:`coroutine` function
   returns, a new :exc:`StopIteration` instance is
   raised, and the value returned by the function is used as the
   :attr:`value` parameter to the constructor of the exception.

   If a generator function defined in the presence of a ``from __future__
   import generator_stop`` directive raises :exc:`StopIteration`, it will be
   converted into a :exc:`RuntimeError` (retaining the :exc:`StopIteration`
   as the new exception's cause).

   .. versionchanged:: 3.3
      Added ``value`` attribute and the ability for generator functions to
      use it to return a value.

   .. versionchanged:: 3.5
      Introduced the RuntimeError transformation.

.. exception:: StopAsyncIteration

   Must be raised by :meth:`__anext__` method of an
   :term:`asynchronous iterator` object to stop the iteration.

   .. versionadded:: 3.5

.. 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:: IndentationError

   Base class for syntax errors related to incorrect indentation.  This is a
   subclass of :exc:`SyntaxError`.


.. exception:: TabError

   Raised when indentation contains an inconsistent use of tabs and spaces.
   This is a subclass of :exc:`IndentationError`.


.. 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.  It 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.  When it is not
   handled, the Python interpreter exits; no stack traceback is printed.  The
   constructor accepts the same optional argument passed to :func:`sys.exit`.
   If the value is an integer, it specifies the system exit status (passed to
   C's :c:func:`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.

   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:`os.fork`).

   .. attribute:: code

      The exit status or error message that is passed to the constructor.
      (Defaults to ``None``.)


.. 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`.

   :exc:`UnicodeError` has attributes that describe the encoding or decoding
   error.  For example, ``err.object[err.start:err.end]`` gives the particular
   invalid input that the codec failed on.

   .. attribute:: encoding

       The name of the encoding that raised the error.

   .. attribute:: reason

       A string describing the specific codec error.

   .. attribute:: object

       The object the codec was attempting to encode or decode.

   .. attribute:: start

       The first index of invalid data in :attr:`object`.

   .. attribute:: end

       The index after the last invalid data in :attr:`object`.


.. 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:: 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 kept for compatibility with previous versions;
starting from Python 3.3, they are aliases of :exc:`OSError`.

.. exception:: EnvironmentError

.. exception:: IOError

.. exception:: WindowsError

   Only available on Windows.


OS exceptions
^^^^^^^^^^^^^

The following exceptions are subclasses of :exc:`OSError`, they get raised
depending on the system error code.

.. exception:: BlockingIOError

   Raised when an operation would block on an object (e.g. socket) set
   for non-blocking operation.
   Corresponds to :c:data:`errno` ``EAGAIN``, ``EALREADY``,
   ``EWOULDBLOCK`` and ``EINPROGRESS``.

   In addition to those of :exc:`OSError`, :exc:`BlockingIOError` can have
   one more attribute:

   .. attribute:: characters_written

      An integer containing the number of characters written to the stream
      before it blocked.  This attribute is available when using the
      buffered I/O classes from the :mod:`io` module.

.. exception:: ChildProcessError

   Raised when an operation on a child process failed.
   Corresponds to :c:data:`errno` ``ECHILD``.

.. exception:: ConnectionError

   A base class for connection-related issues.

   Subclasses are :exc:`BrokenPipeError`, :exc:`ConnectionAbortedError`,
   :exc:`ConnectionRefusedError` and :exc:`ConnectionResetError`.

.. exception:: BrokenPipeError

   A subclass of :exc:`ConnectionError`, raised when trying to write on a
   pipe while the other end has been closed, or trying to write on a socket
   which has been shutdown for writing.
   Corresponds to :c:data:`errno` ``EPIPE`` and ``ESHUTDOWN``.

.. exception:: ConnectionAbortedError

   A subclass of :exc:`ConnectionError`, raised when a connection attempt
   is aborted by the peer.
   Corresponds to :c:data:`errno` ``ECONNABORTED``.

.. exception:: ConnectionRefusedError

   A subclass of :exc:`ConnectionError`, raised when a connection attempt
   is refused by the peer.
   Corresponds to :c:data:`errno` ``ECONNREFUSED``.

.. exception:: ConnectionResetError

   A subclass of :exc:`ConnectionError`, raised when a connection is
   reset by the peer.
   Corresponds to :c:data:`errno` ``ECONNRESET``.

.. exception:: FileExistsError

   Raised when trying to create a file or directory which already exists.
   Corresponds to :c:data:`errno` ``EEXIST``.

.. exception:: FileNotFoundError

   Raised when a file or directory is requested but doesn't exist.
   Corresponds to :c:data:`errno` ``ENOENT``.

.. exception:: InterruptedError

   Raised when a system call is interrupted by an incoming signal.
   Corresponds to :c:data:`errno` :py:data:`~errno.EINTR`.

   .. versionchanged:: 3.5
      Python now retries system calls when a syscall is interrupted by a
      signal, except if the signal handler raises an exception (see :pep:`475`
      for the rationale), instead of raising :exc:`InterruptedError`.

.. exception:: IsADirectoryError

   Raised when a file operation (such as :func:`os.remove`) is requested
   on a directory.
   Corresponds to :c:data:`errno` ``EISDIR``.

.. exception:: NotADirectoryError

   Raised when a directory operation (such as :func:`os.listdir`) is requested
   on something which is not a directory.
   Corresponds to :c:data:`errno` ``ENOTDIR``.

.. exception:: PermissionError

   Raised when trying to run an operation without the adequate access
   rights - for example filesystem permissions.
   Corresponds to :c:data:`errno` ``EACCES`` and ``EPERM``.

.. exception:: ProcessLookupError

   Raised when a given process doesn't exist.
   Corresponds to :c:data:`errno` ``ESRCH``.

.. exception:: TimeoutError

   Raised when a system function timed out at the system level.
   Corresponds to :c:data:`errno` ``ETIMEDOUT``.

.. versionadded:: 3.3
   All the above :exc:`OSError` subclasses were added.


.. seealso::

   :pep:`3151` - Reworking the OS and IO exception hierarchy


Warnings
--------

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:`bytearray`.


.. exception:: ResourceWarning

   Base class for warnings related to resource usage.

   .. versionadded:: 3.2



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

The class hierarchy for built-in exceptions is:

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