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cpython/Doc/library/exceptions.rst
Inada Naoki a3283efd30 Doc: update PendingDeprecationWarning explanation (GH-12837) 
Keep the nudge towards DeprecationWarning, but remove the
"Note" markup and generally shorten the description.

Ref: https://github.com/python/cpython/pull/12505/files#r273978757
2019-04-15 22:40:23 +10:00

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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`
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
Not currently used.
.. 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 the :keyword:`import` statement has troubles trying to
load a module. Also raised when the "from list" in ``from ... import``
has a name that cannot be found.
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:: ModuleNotFoundError
A subclass of :exc:`ImportError` which is raised by :keyword:`import`
when a module could not be located. It is also raised when ``None``
is found in :data:`sys.modules`.
.. versionadded:: 3.6
.. 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, or while the class is being
developed to indicate that the real implementation still needs to be added.
.. note::
It should not be used to indicate that an operator or method is not
meant to be supported at all -- in that case either leave the operator /
method undefined or, if a subclass, set it to :data:`None`.
.. note::
``NotImplementedError`` and ``NotImplemented`` are not interchangeable,
even though they have similar names and purposes. See
:data:`NotImplemented` for details on when to use it.
.. 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 code directly or indirectly raises :exc:`StopIteration`,
it is 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 via
``from __future__ import generator_stop``, see :pep:`479`.
.. versionchanged:: 3.7
Enable :pep:`479` for all code by default: a :exc:`StopIteration`
error raised in a generator is transformed into a :exc:`RuntimeError`.
.. 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.
This exception may be raised by user code to indicate that an attempted
operation on an object is not supported, and is not meant to be. If an object
is meant to support a given operation but has not yet provided an
implementation, :exc:`NotImplementedError` is the proper exception to raise.
Passing arguments of the wrong type (e.g. passing a :class:`list` when an
:class:`int` is expected) should result in a :exc:`TypeError`, but passing
arguments with the wrong value (e.g. a number outside expected boundaries)
should result in a :exc:`ValueError`.
.. 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 an 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
.. _warning-categories-as-exceptions:
Warnings
--------
The following exceptions are used as warning categories; see the
:ref:`warning-categories` documentation for more details.
.. 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 when those warnings are
intended for other Python developers.
.. exception:: PendingDeprecationWarning
Base class for warnings about features which are obsolete and
expected to be deprecated in the future, but are not deprecated
at the moment.
This class is rarely used as emitting a warning about a possible
upcoming deprecation is unusual, and :exc:`DeprecationWarning`
is preferred for already active deprecations.
.. 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 deprecated features when those warnings are
intended for end users of applications that are written in Python.
.. 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. Ignored by the default
warning filters.
.. versionadded:: 3.2
Exception hierarchy
-------------------
The class hierarchy for built-in exceptions is:
.. literalinclude:: ../../Lib/test/exception_hierarchy.txt
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