cpython/Doc/library/exceptions.rst

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.. _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`
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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.
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.. XXX document with_traceback()
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.. 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
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representation of the argument(s) to the instance are returned or the empty
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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
Merged revisions 72506,72525-72526,72551,72558,72616,72654-72655,72689,72745,72750,72802,72812,72822,72824,72826-72827,72833,72876,72890,72923,72946,73026,73042,73045,73047,73065,73068-73069 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72506 | vinay.sajip | 2009-05-09 07:07:17 -0500 (Sat, 09 May 2009) | 1 line Issue #5971: StreamHandler.handleError now swallows IOErrors which occur when trying to print a traceback. ........ r72525 | benjamin.peterson | 2009-05-09 20:38:02 -0500 (Sat, 09 May 2009) | 1 line close file explicitly ........ r72526 | benjamin.peterson | 2009-05-09 21:29:00 -0500 (Sat, 09 May 2009) | 1 line make sure files are closed using the with statement ........ r72551 | benjamin.peterson | 2009-05-10 09:16:47 -0500 (Sun, 10 May 2009) | 1 line use isinstance ........ r72558 | benjamin.peterson | 2009-05-10 18:52:09 -0500 (Sun, 10 May 2009) | 1 line sys.setdefaultencoding() strikes me as a bad example ........ r72616 | benjamin.peterson | 2009-05-13 19:33:10 -0500 (Wed, 13 May 2009) | 1 line importlib.import_module is better these days ........ r72654 | benjamin.peterson | 2009-05-14 17:37:49 -0500 (Thu, 14 May 2009) | 1 line prevent refleaks from threads ........ r72655 | benjamin.peterson | 2009-05-14 17:40:34 -0500 (Thu, 14 May 2009) | 1 line a useful decorator for cleaning up threads ........ r72689 | benjamin.peterson | 2009-05-16 13:44:34 -0500 (Sat, 16 May 2009) | 1 line use skipTest() ........ r72745 | benjamin.peterson | 2009-05-17 09:16:29 -0500 (Sun, 17 May 2009) | 1 line ignore .rst files in sphinx its self ........ r72750 | benjamin.peterson | 2009-05-17 11:59:27 -0500 (Sun, 17 May 2009) | 1 line chop off slash ........ r72802 | georg.brandl | 2009-05-20 13:35:27 -0500 (Wed, 20 May 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72812 | michael.foord | 2009-05-21 17:57:02 -0500 (Thu, 21 May 2009) | 1 line Rename TestCase._result to _resultForDoCleanups to avoid potential clashes in TestCase subclasses. Issue 6072. ........ r72822 | georg.brandl | 2009-05-22 04:33:25 -0500 (Fri, 22 May 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 04:43:17 -0500 (Fri, 22 May 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 04:49:42 -0500 (Fri, 22 May 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 04:50:30 -0500 (Fri, 22 May 2009) | 1 line s/use/call/ ........ r72833 | georg.brandl | 2009-05-22 12:00:17 -0500 (Fri, 22 May 2009) | 1 line #6078: _warnings is a builtin module and has no standard init_warnings function. ........ r72876 | benjamin.peterson | 2009-05-23 15:59:09 -0500 (Sat, 23 May 2009) | 1 line remove mention of old ctypes version ........ r72890 | gregory.p.smith | 2009-05-24 13:00:13 -0500 (Sun, 24 May 2009) | 2 lines add a versionadded tag for set_tunnel ........ r72923 | michael.foord | 2009-05-25 15:36:56 -0500 (Mon, 25 May 2009) | 1 line Make assertSequenceEqual error messages less cryptic, particularly for nested sequences. ........ r72946 | ronald.oussoren | 2009-05-26 13:44:48 -0500 (Tue, 26 May 2009) | 2 lines Fixes issue 6110 ........ r73026 | r.david.murray | 2009-05-29 14:30:27 -0500 (Fri, 29 May 2009) | 3 lines Issue 6141: document that the first item of args is still the command name even when executable is specified. ........ r73042 | benjamin.peterson | 2009-05-29 22:10:52 -0500 (Fri, 29 May 2009) | 1 line no fdatasync on macos ........ r73045 | georg.brandl | 2009-05-30 02:26:04 -0500 (Sat, 30 May 2009) | 1 line #6146: fix markup bug. ........ r73047 | georg.brandl | 2009-05-30 05:33:23 -0500 (Sat, 30 May 2009) | 1 line Fix some more small markup problems. ........ r73065 | antoine.pitrou | 2009-05-30 16:39:25 -0500 (Sat, 30 May 2009) | 3 lines The test for #5330 wasn't correct. ........ r73068 | antoine.pitrou | 2009-05-30 16:45:40 -0500 (Sat, 30 May 2009) | 3 lines Update ACKS ........ r73069 | benjamin.peterson | 2009-05-30 19:42:42 -0500 (Sat, 30 May 2009) | 1 line fix signature ........
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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`.
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.. 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.
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.. 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.)
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.. 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.)
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.. 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
Merged revisions 59275-59303 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! NOTE: The merge does NOT contain the modified file Python/import.c from r59288. I can't get it running. Nick, please check in the PEP 366 manually. !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ........ r59279 | georg.brandl | 2007-12-02 19:17:50 +0100 (Sun, 02 Dec 2007) | 2 lines Fix a sentence I missed before. Do not merge to 3k. ........ r59281 | georg.brandl | 2007-12-02 22:58:54 +0100 (Sun, 02 Dec 2007) | 3 lines Add documentation for PySys_* functions. Written by Charlie Shepherd for GHOP. Also fixes #1245. ........ r59288 | nick.coghlan | 2007-12-03 13:55:17 +0100 (Mon, 03 Dec 2007) | 1 line Implement PEP 366 ........ r59290 | christian.heimes | 2007-12-03 14:47:29 +0100 (Mon, 03 Dec 2007) | 3 lines Applied my patch #1455 with some extra fixes for VS 2005 The new msvc9compiler module supports VS 2005 and VS 2008. I've also fixed build_ext to support PCbuild8 and PCbuild9 and backported my fix for xxmodule.c from py3k. The old code msvccompiler is still in place in case somebody likes to build an extension with VS 2003 or earlier. I've also updated the cygwin compiler module for VS 2005 and VS 2008. It works with VS 2005 but I'm unable to test it with VS 2008. We have to wait for a new version of cygwin. ........ r59291 | christian.heimes | 2007-12-03 14:55:16 +0100 (Mon, 03 Dec 2007) | 1 line Added comment to Misc/NEWS for r59290 ........ r59292 | christian.heimes | 2007-12-03 15:28:04 +0100 (Mon, 03 Dec 2007) | 1 line I followed MA Lemberg's suggestion and added comments to the late initialization of the type slots. ........ r59293 | facundo.batista | 2007-12-03 17:29:52 +0100 (Mon, 03 Dec 2007) | 3 lines Speedup and cleaning of __str__. Thanks Mark Dickinson. ........ r59294 | facundo.batista | 2007-12-03 18:55:00 +0100 (Mon, 03 Dec 2007) | 4 lines Faster _fix function, and some reordering for a more elegant coding. Thanks Mark Dickinson. ........ r59295 | martin.v.loewis | 2007-12-03 20:20:02 +0100 (Mon, 03 Dec 2007) | 5 lines Issue #1727780: Support loading pickles of random.Random objects created on 32-bit systems on 64-bit systems, and vice versa. As a consequence of the change, Random pickles created by Python 2.6 cannot be loaded in Python 2.5. ........ r59297 | facundo.batista | 2007-12-03 20:49:54 +0100 (Mon, 03 Dec 2007) | 3 lines Two small fixes. Issue 1547. ........ r59299 | georg.brandl | 2007-12-03 20:57:02 +0100 (Mon, 03 Dec 2007) | 2 lines #1548: fix apostroph placement. ........ r59300 | christian.heimes | 2007-12-03 21:01:02 +0100 (Mon, 03 Dec 2007) | 3 lines Patch #1537 from Chad Austin Change GeneratorExit's base class from Exception to BaseException (This time I'm applying the patch to the correct sandbox.) ........ r59302 | georg.brandl | 2007-12-03 21:03:46 +0100 (Mon, 03 Dec 2007) | 3 lines Add examples to the xmlrpclib docs. Written for GHOP by Josip Dzolonga. ........
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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.
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.. 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
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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
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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.)
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.. XXX xref to sequences
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.. exception:: KeyError
Raised when a mapping (dictionary) key is not found in the set of existing keys.
.. XXX xref to mapping objects?
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.. 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.
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.. 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
Merged revisions 59921-59932 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r59923 | raymond.hettinger | 2008-01-11 19:04:55 +0100 (Fri, 11 Jan 2008) | 1 line Speed-up and simplify code urlparse's result objects. ........ r59924 | andrew.kuchling | 2008-01-11 20:33:24 +0100 (Fri, 11 Jan 2008) | 1 line Bug #1790: update link; remove outdated paragraph ........ r59925 | thomas.heller | 2008-01-11 20:34:06 +0100 (Fri, 11 Jan 2008) | 5 lines Raise an error instead of crashing with a segfault when a NULL function pointer is called. Will backport to release25-maint. ........ r59927 | thomas.heller | 2008-01-11 21:29:19 +0100 (Fri, 11 Jan 2008) | 4 lines Fix a potential 'SystemError: NULL result without error'. NULL may be a valid return value from PyLong_AsVoidPtr. Will backport to release25-maint. ........ r59928 | raymond.hettinger | 2008-01-12 00:25:18 +0100 (Sat, 12 Jan 2008) | 1 line Update the opcode docs for STORE_MAP and BUILD_MAP ........ r59929 | mark.dickinson | 2008-01-12 02:56:00 +0100 (Sat, 12 Jan 2008) | 4 lines Issue 1780: Allow leading and trailing whitespace in Decimal constructor, when constructing from a string. Disallow trailing newlines in Context.create_decimal. ........ r59930 | georg.brandl | 2008-01-12 11:53:29 +0100 (Sat, 12 Jan 2008) | 3 lines Move OSError docs to exceptions doc, remove obsolete descriptions from os docs, rework posix docs. ........ r59931 | georg.brandl | 2008-01-12 14:47:57 +0100 (Sat, 12 Jan 2008) | 3 lines Patch #1700288: Method cache optimization, by Armin Rigo, ported to 2.6 by Kevin Jacobs. ........ r59932 | georg.brandl | 2008-01-12 17:11:09 +0100 (Sat, 12 Jan 2008) | 2 lines Fix editing glitch. ........
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.. 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.
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.. 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
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: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.
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.. 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.
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.. 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
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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.
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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`.
Merged revisions 69576,69579-69580,69589,69619-69620,69633,69703-69704,69728-69730 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r69576 | georg.brandl | 2009-02-13 04:56:50 -0600 (Fri, 13 Feb 2009) | 1 line #1661108: note that urlsafe encoded string can contain "=". ........ r69579 | georg.brandl | 2009-02-13 05:06:59 -0600 (Fri, 13 Feb 2009) | 2 lines Fix warnings GCC emits where the argument of PyErr_Format is a single variable. ........ r69580 | georg.brandl | 2009-02-13 05:10:04 -0600 (Fri, 13 Feb 2009) | 2 lines Fix warnings GCC emits where the argument of PyErr_Format is a single variable. ........ r69589 | martin.v.loewis | 2009-02-13 14:11:34 -0600 (Fri, 13 Feb 2009) | 2 lines Move amd64 properties further to the top, so that they override the linker options correctly. ........ r69619 | benjamin.peterson | 2009-02-14 11:00:51 -0600 (Sat, 14 Feb 2009) | 1 line this needn't be a shebang line ........ r69620 | georg.brandl | 2009-02-14 11:01:36 -0600 (Sat, 14 Feb 2009) | 1 line #5179: don't leak PIPE fds when child execution fails. ........ r69633 | hirokazu.yamamoto | 2009-02-15 03:19:48 -0600 (Sun, 15 Feb 2009) | 1 line Fixed typo. ........ r69703 | raymond.hettinger | 2009-02-16 16:42:54 -0600 (Mon, 16 Feb 2009) | 3 lines Issue 5229: Documentation for super() neglects to say what super() actually does ........ r69704 | raymond.hettinger | 2009-02-16 17:00:25 -0600 (Mon, 16 Feb 2009) | 1 line Add explanation for super(type1, type2). ........ r69728 | georg.brandl | 2009-02-17 18:22:55 -0600 (Tue, 17 Feb 2009) | 2 lines #5297: fix example. ........ r69729 | georg.brandl | 2009-02-17 18:25:13 -0600 (Tue, 17 Feb 2009) | 2 lines #5296: sequence -> iterable. ........ r69730 | georg.brandl | 2009-02-17 18:31:36 -0600 (Tue, 17 Feb 2009) | 2 lines #5268: mention VMSError. ........
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.. exception:: VMSError
Only available on VMS. Raised when a VMS-specific error occurs.
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.. 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.
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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`.
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Exception hierarchy
-------------------
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The class hierarchy for built-in exceptions is:
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.. literalinclude:: ../../Lib/test/exception_hierarchy.txt