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

In Python, all exceptions must be instances of a class that derives
from ``BaseException``.  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 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 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*.

When raising (or re-raising) an exception in an ``except`` clause
``__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
``from`` with ``raise``:

   raise new_exc from original_exc

The expression following ``from`` must be an exception or ``None``. It
will be set as ``__cause__`` on the raised exception. Setting
``__cause__`` also implicitly sets the ``__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 ``KeyError`` to ``AttributeError``, while
leaving the old exception available in ``__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 ``__cause__`` is always shown when present. An
implicitly chained exception in ``__context__`` is shown only if
``__cause__`` is ``None`` and ``__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 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()`` 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.

   args

      The tuple of arguments given to the exception constructor.  Some
      built-in exceptions (like ``IOError``) 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.

   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 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 BufferError

   Raised when a *buffer* related operation cannot be performed.

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()``.


Concrete exceptions
===================

The following exceptions are the exceptions that are usually 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 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.

   The ``name`` and ``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.

   Changed in version 3.3: Added the ``name`` and ``path`` attributes.

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 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).  Often a subclass of ``OSError`` will actually be raised
   as described in OS exceptions below.  The ``errno`` attribute is a
   numeric error code from the C variable ``errno``.

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

   Under all platforms, the ``strerror`` attribute is the
   corresponding error message as provided by the operating system (as
   formatted by the C functions ``perror()`` under POSIX, and
   ``FormatMessage()`` Windows).

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

   Changed in version 3.3: ``EnvironmentError``, ``IOError``,
   ``WindowsError``, ``VMSError``, ``socket.error``, ``select.error``
   and ``mmap.error`` have been merged into ``OSError``.

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 built-in function ``next()`` and an *iterator*'s
   ``__next__()`` method to signal that there are no further items
   produced by the iterator.

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

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

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

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 IndentationError

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

exception exception TabError

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

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``.

   ``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.

   encoding

      The name of the encoding that raised the error.

   reason

      A string describing the specific codec error.

   object

      The object the codec was attempting to encode or decode.

   start

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

   end

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

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 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 ``OSError``.

exception exception EnvironmentError

exception exception IOError

exception exception VMSError

   Only available on VMS.

exception exception WindowsError

   Only available on Windows.


OS exceptions
-------------

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

exception exception BlockingIOError

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

   In addition to those of ``OSError``, ``BlockingIOError`` can have
   one more 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 ``io`` module.

exception exception ChildProcessError

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

exception exception ConnectionError

   A base class for connection-related issues.

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

exception exception BrokenPipeError

   A subclass of ``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
   ``errno`` ``EPIPE`` and ``ESHUTDOWN``.

exception exception ConnectionAbortedError

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

exception exception ConnectionRefusedError

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

exception exception ConnectionResetError

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

exception exception FileExistsError

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

exception exception FileNotFoundError

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

exception exception InterruptedError

   Raised when a system call is interrupted by an incoming signal.
   Corresponds to ``errno`` ``EINTR``.

exception exception IsADirectoryError

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

exception exception NotADirectoryError

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

exception exception PermissionError

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

exception exception ProcessLookupError

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

exception exception TimeoutError

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

New in version 3.3: All the above ``OSError`` subclasses were added.

See also:

   **PEP 3151** - Reworking the OS and IO exception hierarchy


Warnings
========

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 exception ResourceWarning

   Base class for warnings related to resource usage.

   New in version 3.2.


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

The class hierarchy for built-in exceptions is:

   BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
         +-- StopIteration
         +-- ArithmeticError
         |    +-- FloatingPointError
         |    +-- OverflowError
         |    +-- ZeroDivisionError
         +-- AssertionError
         +-- AttributeError
         +-- BufferError
         +-- EOFError
         +-- ImportError
         +-- LookupError
         |    +-- IndexError
         |    +-- KeyError
         +-- MemoryError
         +-- NameError
         |    +-- UnboundLocalError
         +-- OSError
         |    +-- BlockingIOError
         |    +-- ChildProcessError
         |    +-- ConnectionError
         |    |    +-- BrokenPipeError
         |    |    +-- ConnectionAbortedError
         |    |    +-- ConnectionRefusedError
         |    |    +-- ConnectionResetError
         |    +-- FileExistsError
         |    +-- FileNotFoundError
         |    +-- InterruptedError
         |    +-- IsADirectoryError
         |    +-- NotADirectoryError
         |    +-- PermissionError
         |    +-- ProcessLookupError
         |    +-- TimeoutError
         +-- ReferenceError
         +-- RuntimeError
         |    +-- NotImplementedError
         +-- SyntaxError
         |    +-- IndentationError
         |         +-- TabError
         +-- SystemError
         +-- TypeError
         +-- ValueError
         |    +-- UnicodeError
         |         +-- UnicodeDecodeError
         |         +-- UnicodeEncodeError
         |         +-- UnicodeTranslateError
         +-- Warning
              +-- DeprecationWarning
              +-- PendingDeprecationWarning
              +-- RuntimeWarning
              +-- SyntaxWarning
              +-- UserWarning
              +-- FutureWarning
              +-- ImportWarning
              +-- UnicodeWarning
              +-- BytesWarning
              +-- ResourceWarning
