Doc/library/warnings.rst - platform/external/python/cpython3 - Gitiles

 :mod:`warnings` --- Warning control
 ===================================

 .. module:: warnings
    :synopsis: Issue warning messages and control their disposition.

 **Source code:** :source:`Lib/warnings.py`

 .. index:: single: warnings

 --------------

 Warning messages are typically issued in situations where it is useful to alert
 the user of some condition in a program, where that condition (normally) doesn't
 warrant raising an exception and terminating the program.  For example, one
 might want to issue a warning when a program uses an obsolete module.

 Python programmers issue warnings by calling the :func:`warn` function defined
 in this module.  (C programmers use :c:func:`PyErr_WarnEx`; see
 :ref:`exceptionhandling` for details).

 Warning messages are normally written to ``sys.stderr``, but their disposition
 can be changed flexibly, from ignoring all warnings to turning them into
 exceptions.  The disposition of warnings can vary based on the warning category
 (see below), the text of the warning message, and the source location where it
 is issued.  Repetitions of a particular warning for the same source location are
 typically suppressed.

 There are two stages in warning control: first, each time a warning is issued, a
 determination is made whether a message should be issued or not; next, if a
 message is to be issued, it is formatted and printed using a user-settable hook.

 The determination whether to issue a warning message is controlled by the
 warning filter, which is a sequence of matching rules and actions. Rules can be
 added to the filter by calling :func:`filterwarnings` and reset to its default
 state by calling :func:`resetwarnings`.

 The printing of warning messages is done by calling :func:`showwarning`, which
 may be overridden; the default implementation of this function formats the
 message by calling :func:`formatwarning`, which is also available for use by
 custom implementations.

 .. seealso::
    :func:`logging.captureWarnings` allows you to handle all warnings with
    the standard logging infrastructure.


 .. _warning-categories:

 Warning Categories
 ------------------

 There are a number of built-in exceptions that represent warning categories.
 This categorization is useful to be able to filter out groups of warnings.  The
 following warnings category classes are currently defined:

 .. tabularcolumns:: |l|p{0.6\linewidth}|

 +----------------------------------+-----------------------------------------------+
 | Class                            | Description                                   |
 +==================================+===============================================+
 | :exc:`Warning`                   | This is the base class of all warning         |
 |                                  | category classes.  It is a subclass of        |
 |                                  | :exc:`Exception`.                             |
 +----------------------------------+-----------------------------------------------+
 | :exc:`UserWarning`               | The default category for :func:`warn`.        |
 +----------------------------------+-----------------------------------------------+
 | :exc:`DeprecationWarning`        | Base category for warnings about deprecated   |
 |                                  | features (ignored by default).                |
 +----------------------------------+-----------------------------------------------+
 | :exc:`SyntaxWarning`             | Base category for warnings about dubious      |
 |                                  | syntactic features.                           |
 +----------------------------------+-----------------------------------------------+
 | :exc:`RuntimeWarning`            | Base category for warnings about dubious      |
 |                                  | runtime features.                             |
 +----------------------------------+-----------------------------------------------+
 | :exc:`FutureWarning`             | Base category for warnings about constructs   |
 |                                  | that will change semantically in the future.  |
 +----------------------------------+-----------------------------------------------+
 | :exc:`PendingDeprecationWarning` | Base category for warnings about features     |
 |                                  | that will be deprecated in the future         |
 |                                  | (ignored by default).                         |
 +----------------------------------+-----------------------------------------------+
 | :exc:`ImportWarning`             | Base category for warnings triggered during   |
 |                                  | the process of importing a module (ignored by |
 |                                  | default).                                     |
 +----------------------------------+-----------------------------------------------+
 | :exc:`UnicodeWarning`            | Base category for warnings related to         |
 |                                  | Unicode.                                      |
 +----------------------------------+-----------------------------------------------+
 | :exc:`BytesWarning`              | Base category for warnings related to         |
 |                                  | :class:`bytes` and :class:`bytearray`.        |
 +----------------------------------+-----------------------------------------------+
 | :exc:`ResourceWarning`           | Base category for warnings related to         |
 |                                  | resource usage.                               |
 +----------------------------------+-----------------------------------------------+


 While these are technically built-in exceptions, they are documented here,
 because conceptually they belong to the warnings mechanism.

 User code can define additional warning categories by subclassing one of the
 standard warning categories.  A warning category must always be a subclass of
 the :exc:`Warning` class.


 .. _warning-filter:

 The Warnings Filter
 -------------------

 The warnings filter controls whether warnings are ignored, displayed, or turned
 into errors (raising an exception).

 Conceptually, the warnings filter maintains an ordered list of filter
 specifications; any specific warning is matched against each filter
 specification in the list in turn until a match is found; the match determines
 the disposition of the match.  Each entry is a tuple of the form (*action*,
 *message*, *category*, *module*, *lineno*), where:

 * *action* is one of the following strings:

   +---------------+----------------------------------------------+
   | Value         | Disposition                                  |
   +===============+==============================================+
   | ``"error"``   | turn matching warnings into exceptions       |
   +---------------+----------------------------------------------+
   | ``"ignore"``  | never print matching warnings                |
   +---------------+----------------------------------------------+
   | ``"always"``  | always print matching warnings               |
   +---------------+----------------------------------------------+
   | ``"default"`` | print the first occurrence of matching       |
   |               | warnings for each location where the warning |
   |               | is issued                                    |
   +---------------+----------------------------------------------+
   | ``"module"``  | print the first occurrence of matching       |
   |               | warnings for each module where the warning   |
   |               | is issued                                    |
   +---------------+----------------------------------------------+
   | ``"once"``    | print only the first occurrence of matching  |
   |               | warnings, regardless of location             |
   +---------------+----------------------------------------------+

 * *message* is a string containing a regular expression that the start of
   the warning message must match.  The expression is compiled to always be
   case-insensitive.

 * *category* is a class (a subclass of :exc:`Warning`) of which the warning
   category must be a subclass in order to match.

 * *module* is a string containing a regular expression that the module name must
   match.  The expression is compiled to be case-sensitive.

 * *lineno* is an integer that the line number where the warning occurred must
   match, or ``0`` to match all line numbers.

 Since the :exc:`Warning` class is derived from the built-in :exc:`Exception`
 class, to turn a warning into an error we simply raise ``category(message)``.

 The warnings filter is initialized by :option:`-W` options passed to the Python
 interpreter command line.  The interpreter saves the arguments for all
 :option:`-W` options without interpretation in ``sys.warnoptions``; the
 :mod:`warnings` module parses these when it is first imported (invalid options
 are ignored, after printing a message to ``sys.stderr``).


 Default Warning Filters
 ~~~~~~~~~~~~~~~~~~~~~~~

 By default, Python installs several warning filters, which can be overridden by
 the command-line options passed to :option:`-W` and calls to
 :func:`filterwarnings`.

 * :exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, and
   :exc:`ImportWarning` are ignored.

 * :exc:`BytesWarning` is ignored unless the :option:`-b` option is given once or
   twice; in this case this warning is either printed (``-b``) or turned into an
   exception (``-bb``).

 * :exc:`ResourceWarning` is ignored unless Python was built in debug mode.

 .. versionchanged:: 3.2
    :exc:`DeprecationWarning` is now ignored by default in addition to
    :exc:`PendingDeprecationWarning`.


 .. _warning-suppress:

 Temporarily Suppressing Warnings
 --------------------------------

 If you are using code that you know will raise a warning, such as a deprecated
 function, but do not want to see the warning, then it is possible to suppress
 the warning using the :class:`catch_warnings` context manager::

     import warnings

     def fxn():
         warnings.warn("deprecated", DeprecationWarning)

     with warnings.catch_warnings():
         warnings.simplefilter("ignore")
         fxn()

 While within the context manager all warnings will simply be ignored. This
 allows you to use known-deprecated code without having to see the warning while
 not suppressing the warning for other code that might not be aware of its use
 of deprecated code.  Note: this can only be guaranteed in a single-threaded
 application. If two or more threads use the :class:`catch_warnings` context
 manager at the same time, the behavior is undefined.


 .. _warning-testing:

 Testing Warnings
 ----------------

 To test warnings raised by code, use the :class:`catch_warnings` context
 manager. With it you can temporarily mutate the warnings filter to facilitate
 your testing. For instance, do the following to capture all raised warnings to
 check::

     import warnings

     def fxn():
         warnings.warn("deprecated", DeprecationWarning)

     with warnings.catch_warnings(record=True) as w:
         # Cause all warnings to always be triggered.
         warnings.simplefilter("always")
         # Trigger a warning.
         fxn()
         # Verify some things
         assert len(w) == 1
         assert issubclass(w[-1].category, DeprecationWarning)
         assert "deprecated" in str(w[-1].message)

 One can also cause all warnings to be exceptions by using ``error`` instead of
 ``always``. One thing to be aware of is that if a warning has already been
 raised because of a ``once``/``default`` rule, then no matter what filters are
 set the warning will not be seen again unless the warnings registry related to
 the warning has been cleared.

 Once the context manager exits, the warnings filter is restored to its state
 when the context was entered. This prevents tests from changing the warnings
 filter in unexpected ways between tests and leading to indeterminate test
 results. The :func:`showwarning` function in the module is also restored to
 its original value.  Note: this can only be guaranteed in a single-threaded
 application. If two or more threads use the :class:`catch_warnings` context
 manager at the same time, the behavior is undefined.

 When testing multiple operations that raise the same kind of warning, it
 is important to test them in a manner that confirms each operation is raising
 a new warning (e.g. set warnings to be raised as exceptions and check the
 operations raise exceptions, check that the length of the warning list
 continues to increase after each operation, or else delete the previous
 entries from the warnings list before each new operation).


 .. _warning-ignored:

 Updating Code For New Versions of Python
 ----------------------------------------

 Warnings that are only of interest to the developer are ignored by default. As
 such you should make sure to test your code with typically ignored warnings
 made visible. You can do this from the command-line by passing :option:`-Wd <-W>`
 to the interpreter (this is shorthand for :option:`!-W default`).  This enables
 default handling for all warnings, including those that are ignored by default.
 To change what action is taken for encountered warnings you simply change what
 argument is passed to :option:`-W`, e.g. :option:`!-W error`. See the
 :option:`-W` flag for more details on what is possible.

 To programmatically do the same as :option:`!-Wd`, use::

   warnings.simplefilter('default')

 Make sure to execute this code as soon as possible. This prevents the
 registering of what warnings have been raised from unexpectedly influencing how
 future warnings are treated.

 Having certain warnings ignored by default is done to prevent a user from
 seeing warnings that are only of interest to the developer. As you do not
 necessarily have control over what interpreter a user uses to run their code,
 it is possible that a new version of Python will be released between your
 release cycles.  The new interpreter release could trigger new warnings in your
 code that were not there in an older interpreter, e.g.
 :exc:`DeprecationWarning` for a module that you are using. While you as a
 developer want to be notified that your code is using a deprecated module, to a
 user this information is essentially noise and provides no benefit to them.

 The :mod:`unittest` module has been also updated to use the ``'default'``
 filter while running tests.


 .. _warning-functions:

 Available Functions
 -------------------


 .. function:: warn(message, category=None, stacklevel=1, source=None)

    Issue a warning, or maybe ignore it or raise an exception.  The *category*
    argument, if given, must be a warning category class (see above); it defaults to
    :exc:`UserWarning`.  Alternatively *message* can be a :exc:`Warning` instance,
    in which case *category* will be ignored and ``message.__class__`` will be used.
    In this case the message text will be ``str(message)``. This function raises an
    exception if the particular warning issued is changed into an error by the
    warnings filter see above.  The *stacklevel* argument can be used by wrapper
    functions written in Python, like this::

       def deprecation(message):
           warnings.warn(message, DeprecationWarning, stacklevel=2)

    This makes the warning refer to :func:`deprecation`'s caller, rather than to the
    source of :func:`deprecation` itself (since the latter would defeat the purpose
    of the warning message).

    *source*, if supplied, is the destroyed object which emitted a
    :exc:`ResourceWarning`.

    .. versionchanged:: 3.6
       Added *source* parameter.


 .. function:: warn_explicit(message, category, filename, lineno, module=None, registry=None, module_globals=None, source=None)

    This is a low-level interface to the functionality of :func:`warn`, passing in
    explicitly the message, category, filename and line number, and optionally the
    module name and the registry (which should be the ``__warningregistry__``
    dictionary of the module).  The module name defaults to the filename with
    ``.py`` stripped; if no registry is passed, the warning is never suppressed.
    *message* must be a string and *category* a subclass of :exc:`Warning` or
    *message* may be a :exc:`Warning` instance, in which case *category* will be
    ignored.

    *module_globals*, if supplied, should be the global namespace in use by the code
    for which the warning is issued.  (This argument is used to support displaying
    source for modules found in zipfiles or other non-filesystem import
    sources).

    *source*, if supplied, is the destroyed object which emitted a
    :exc:`ResourceWarning`.

    .. versionchanged:: 3.6
       Add the *source* parameter.


 .. function:: showwarning(message, category, filename, lineno, file=None, line=None)

    Write a warning to a file.  The default implementation calls
    ``formatwarning(message, category, filename, lineno, line)`` and writes the
    resulting string to *file*, which defaults to ``sys.stderr``.  You may replace
    this function with any callable by assigning to ``warnings.showwarning``.
    *line* is a line of source code to be included in the warning
    message; if *line* is not supplied, :func:`showwarning` will
    try to read the line specified by *filename* and *lineno*.


 .. function:: formatwarning(message, category, filename, lineno, line=None)

    Format a warning the standard way.  This returns a string which may contain
    embedded newlines and ends in a newline.  *line* is a line of source code to
    be included in the warning message; if *line* is not supplied,
    :func:`formatwarning` will try to read the line specified by *filename* and
    *lineno*.


 .. function:: filterwarnings(action, message='', category=Warning, module='', lineno=0, append=False)

    Insert an entry into the list of :ref:`warnings filter specifications
    <warning-filter>`.  The entry is inserted at the front by default; if
    *append* is true, it is inserted at the end.  This checks the types of the
    arguments, compiles the *message* and *module* regular expressions, and
    inserts them as a tuple in the list of warnings filters.  Entries closer to
    the front of the list override entries later in the list, if both match a
    particular warning.  Omitted arguments default to a value that matches
    everything.


 .. function:: simplefilter(action, category=Warning, lineno=0, append=False)

    Insert a simple entry into the list of :ref:`warnings filter specifications
    <warning-filter>`.  The meaning of the function parameters is as for
    :func:`filterwarnings`, but regular expressions are not needed as the filter
    inserted always matches any message in any module as long as the category and
    line number match.


 .. function:: resetwarnings()

    Reset the warnings filter.  This discards the effect of all previous calls to
    :func:`filterwarnings`, including that of the :option:`-W` command line options
    and calls to :func:`simplefilter`.


 Available Context Managers
 --------------------------

 .. class:: catch_warnings(\*, record=False, module=None)

     A context manager that copies and, upon exit, restores the warnings filter
     and the :func:`showwarning` function.
     If the *record* argument is :const:`False` (the default) the context manager
     returns :class:`None` on entry. If *record* is :const:`True`, a list is
     returned that is progressively populated with objects as seen by a custom
     :func:`showwarning` function (which also suppresses output to ``sys.stdout``).
     Each object in the list has attributes with the same names as the arguments to
     :func:`showwarning`.

     The *module* argument takes a module that will be used instead of the
     module returned when you import :mod:`warnings` whose filter will be
     protected. This argument exists primarily for testing the :mod:`warnings`
     module itself.

     .. note::

         The :class:`catch_warnings` manager works by replacing and
         then later restoring the module's
         :func:`showwarning` function and internal list of filter
         specifications.  This means the context manager is modifying
         global state and therefore is not thread-safe.
	:mod:`warnings` --- Warning control
	===================================

	.. module:: warnings
	:synopsis: Issue warning messages and control their disposition.

	Source code: :source:`Lib/warnings.py`

	.. index:: single: warnings

	--------------

	Warning messages are typically issued in situations where it is useful to alert
	the user of some condition in a program, where that condition (normally) doesn't
	warrant raising an exception and terminating the program. For example, one
	might want to issue a warning when a program uses an obsolete module.

	Python programmers issue warnings by calling the :func:`warn` function defined
	in this module. (C programmers use :c:func:`PyErr_WarnEx`; see
	:ref:`exceptionhandling` for details).

	Warning messages are normally written to ``sys.stderr``, but their disposition
	can be changed flexibly, from ignoring all warnings to turning them into
	exceptions. The disposition of warnings can vary based on the warning category
	(see below), the text of the warning message, and the source location where it
	is issued. Repetitions of a particular warning for the same source location are
	typically suppressed.

	There are two stages in warning control: first, each time a warning is issued, a
	determination is made whether a message should be issued or not; next, if a
	message is to be issued, it is formatted and printed using a user-settable hook.

	The determination whether to issue a warning message is controlled by the
	warning filter, which is a sequence of matching rules and actions. Rules can be
	added to the filter by calling :func:`filterwarnings` and reset to its default
	state by calling :func:`resetwarnings`.

	The printing of warning messages is done by calling :func:`showwarning`, which
	may be overridden; the default implementation of this function formats the
	message by calling :func:`formatwarning`, which is also available for use by
	custom implementations.

	.. seealso::
	:func:`logging.captureWarnings` allows you to handle all warnings with
	the standard logging infrastructure.


	.. _warning-categories:

	Warning Categories
	------------------

	There are a number of built-in exceptions that represent warning categories.
	This categorization is useful to be able to filter out groups of warnings. The
	following warnings category classes are currently defined:

	.. tabularcolumns:: \|l\|p{0.6\linewidth}\|

	+----------------------------------+-----------------------------------------------+
	\| Class \| Description \|
	+==================================+===============================================+
	\| :exc:`Warning` \| This is the base class of all warning \|
	\| \| category classes. It is a subclass of \|
	\| \| :exc:`Exception`. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`UserWarning` \| The default category for :func:`warn`. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`DeprecationWarning` \| Base category for warnings about deprecated \|
	\| \| features (ignored by default). \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`SyntaxWarning` \| Base category for warnings about dubious \|
	\| \| syntactic features. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`RuntimeWarning` \| Base category for warnings about dubious \|
	\| \| runtime features. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`FutureWarning` \| Base category for warnings about constructs \|
	\| \| that will change semantically in the future. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`PendingDeprecationWarning` \| Base category for warnings about features \|
	\| \| that will be deprecated in the future \|
	\| \| (ignored by default). \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`ImportWarning` \| Base category for warnings triggered during \|
	\| \| the process of importing a module (ignored by \|
	\| \| default). \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`UnicodeWarning` \| Base category for warnings related to \|
	\| \| Unicode. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`BytesWarning` \| Base category for warnings related to \|
	\| \| :class:`bytes` and :class:`bytearray`. \|
	+----------------------------------+-----------------------------------------------+
	\| :exc:`ResourceWarning` \| Base category for warnings related to \|
	\| \| resource usage. \|
	+----------------------------------+-----------------------------------------------+


	While these are technically built-in exceptions, they are documented here,
	because conceptually they belong to the warnings mechanism.

	User code can define additional warning categories by subclassing one of the
	standard warning categories. A warning category must always be a subclass of
	the :exc:`Warning` class.


	.. _warning-filter:

	The Warnings Filter
	-------------------

	The warnings filter controls whether warnings are ignored, displayed, or turned
	into errors (raising an exception).

	Conceptually, the warnings filter maintains an ordered list of filter
	specifications; any specific warning is matched against each filter
	specification in the list in turn until a match is found; the match determines
	the disposition of the match. Each entry is a tuple of the form (action,
	message, category, module, lineno), where:

	* action is one of the following strings:

	+---------------+----------------------------------------------+
	\| Value \| Disposition \|
	+===============+==============================================+
	\| ``"error"`` \| turn matching warnings into exceptions \|
	+---------------+----------------------------------------------+
	\| ``"ignore"`` \| never print matching warnings \|
	+---------------+----------------------------------------------+
	\| ``"always"`` \| always print matching warnings \|
	+---------------+----------------------------------------------+
	\| ``"default"`` \| print the first occurrence of matching \|
	\| \| warnings for each location where the warning \|
	\| \| is issued \|
	+---------------+----------------------------------------------+
	\| ``"module"`` \| print the first occurrence of matching \|
	\| \| warnings for each module where the warning \|
	\| \| is issued \|
	+---------------+----------------------------------------------+
	\| ``"once"`` \| print only the first occurrence of matching \|
	\| \| warnings, regardless of location \|
	+---------------+----------------------------------------------+

	* message is a string containing a regular expression that the start of
	the warning message must match. The expression is compiled to always be
	case-insensitive.

	* category is a class (a subclass of :exc:`Warning`) of which the warning
	category must be a subclass in order to match.

	* module is a string containing a regular expression that the module name must
	match. The expression is compiled to be case-sensitive.

	* lineno is an integer that the line number where the warning occurred must
	match, or ``0`` to match all line numbers.

	Since the :exc:`Warning` class is derived from the built-in :exc:`Exception`
	class, to turn a warning into an error we simply raise ``category(message)``.

	The warnings filter is initialized by :option:`-W` options passed to the Python
	interpreter command line. The interpreter saves the arguments for all
	:option:`-W` options without interpretation in ``sys.warnoptions``; the
	:mod:`warnings` module parses these when it is first imported (invalid options
	are ignored, after printing a message to ``sys.stderr``).


	Default Warning Filters
	~~~~~~~~~~~~~~~~~~~~~~~

	By default, Python installs several warning filters, which can be overridden by
	the command-line options passed to :option:`-W` and calls to
	:func:`filterwarnings`.

	* :exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, and
	:exc:`ImportWarning` are ignored.

	* :exc:`BytesWarning` is ignored unless the :option:`-b` option is given once or
	twice; in this case this warning is either printed (``-b``) or turned into an
	exception (``-bb``).

	* :exc:`ResourceWarning` is ignored unless Python was built in debug mode.

	.. versionchanged:: 3.2
	:exc:`DeprecationWarning` is now ignored by default in addition to
	:exc:`PendingDeprecationWarning`.


	.. _warning-suppress:

	Temporarily Suppressing Warnings
	--------------------------------

	If you are using code that you know will raise a warning, such as a deprecated
	function, but do not want to see the warning, then it is possible to suppress
	the warning using the :class:`catch_warnings` context manager::

	import warnings

	def fxn():
	warnings.warn("deprecated", DeprecationWarning)

	with warnings.catch_warnings():
	warnings.simplefilter("ignore")
	fxn()

	While within the context manager all warnings will simply be ignored. This
	allows you to use known-deprecated code without having to see the warning while
	not suppressing the warning for other code that might not be aware of its use
	of deprecated code. Note: this can only be guaranteed in a single-threaded
	application. If two or more threads use the :class:`catch_warnings` context
	manager at the same time, the behavior is undefined.



	.. _warning-testing:

	Testing Warnings
	----------------

	To test warnings raised by code, use the :class:`catch_warnings` context
	manager. With it you can temporarily mutate the warnings filter to facilitate
	your testing. For instance, do the following to capture all raised warnings to
	check::

	import warnings

	def fxn():
	warnings.warn("deprecated", DeprecationWarning)

	with warnings.catch_warnings(record=True) as w:
	# Cause all warnings to always be triggered.
	warnings.simplefilter("always")
	# Trigger a warning.
	fxn()
	# Verify some things
	assert len(w) == 1
	assert issubclass(w[-1].category, DeprecationWarning)
	assert "deprecated" in str(w[-1].message)

	One can also cause all warnings to be exceptions by using ``error`` instead of
	``always``. One thing to be aware of is that if a warning has already been
	raised because of a ``once``/``default`` rule, then no matter what filters are
	set the warning will not be seen again unless the warnings registry related to
	the warning has been cleared.

	Once the context manager exits, the warnings filter is restored to its state
	when the context was entered. This prevents tests from changing the warnings
	filter in unexpected ways between tests and leading to indeterminate test
	results. The :func:`showwarning` function in the module is also restored to
	its original value. Note: this can only be guaranteed in a single-threaded
	application. If two or more threads use the :class:`catch_warnings` context
	manager at the same time, the behavior is undefined.

	When testing multiple operations that raise the same kind of warning, it
	is important to test them in a manner that confirms each operation is raising
	a new warning (e.g. set warnings to be raised as exceptions and check the
	operations raise exceptions, check that the length of the warning list
	continues to increase after each operation, or else delete the previous
	entries from the warnings list before each new operation).


	.. _warning-ignored:

	Updating Code For New Versions of Python
	----------------------------------------

	Warnings that are only of interest to the developer are ignored by default. As
	such you should make sure to test your code with typically ignored warnings
	made visible. You can do this from the command-line by passing :option:`-Wd <-W>`
	to the interpreter (this is shorthand for :option:`!-W default`). This enables
	default handling for all warnings, including those that are ignored by default.
	To change what action is taken for encountered warnings you simply change what
	argument is passed to :option:`-W`, e.g. :option:`!-W error`. See the
	:option:`-W` flag for more details on what is possible.

	To programmatically do the same as :option:`!-Wd`, use::

	warnings.simplefilter('default')

	Make sure to execute this code as soon as possible. This prevents the
	registering of what warnings have been raised from unexpectedly influencing how
	future warnings are treated.

	Having certain warnings ignored by default is done to prevent a user from
	seeing warnings that are only of interest to the developer. As you do not
	necessarily have control over what interpreter a user uses to run their code,
	it is possible that a new version of Python will be released between your
	release cycles. The new interpreter release could trigger new warnings in your
	code that were not there in an older interpreter, e.g.
	:exc:`DeprecationWarning` for a module that you are using. While you as a
	developer want to be notified that your code is using a deprecated module, to a
	user this information is essentially noise and provides no benefit to them.

	The :mod:`unittest` module has been also updated to use the ``'default'``
	filter while running tests.


	.. _warning-functions:

	Available Functions
	-------------------


	.. function:: warn(message, category=None, stacklevel=1, source=None)

	Issue a warning, or maybe ignore it or raise an exception. The category
	argument, if given, must be a warning category class (see above); it defaults to
	:exc:`UserWarning`. Alternatively message can be a :exc:`Warning` instance,
	in which case category will be ignored and ``message.__class__`` will be used.
	In this case the message text will be ``str(message)``. This function raises an
	exception if the particular warning issued is changed into an error by the
	warnings filter see above. The stacklevel argument can be used by wrapper
	functions written in Python, like this::

	def deprecation(message):
	warnings.warn(message, DeprecationWarning, stacklevel=2)

	This makes the warning refer to :func:`deprecation`'s caller, rather than to the
	source of :func:`deprecation` itself (since the latter would defeat the purpose
	of the warning message).

	source, if supplied, is the destroyed object which emitted a
	:exc:`ResourceWarning`.

	.. versionchanged:: 3.6
	Added source parameter.


	.. function:: warn_explicit(message, category, filename, lineno, module=None, registry=None, module_globals=None, source=None)

	This is a low-level interface to the functionality of :func:`warn`, passing in
	explicitly the message, category, filename and line number, and optionally the
	module name and the registry (which should be the ``__warningregistry__``
	dictionary of the module). The module name defaults to the filename with
	``.py`` stripped; if no registry is passed, the warning is never suppressed.
	message must be a string and category a subclass of :exc:`Warning` or
	message may be a :exc:`Warning` instance, in which case category will be
	ignored.

	module_globals, if supplied, should be the global namespace in use by the code
	for which the warning is issued. (This argument is used to support displaying
	source for modules found in zipfiles or other non-filesystem import
	sources).

	source, if supplied, is the destroyed object which emitted a
	:exc:`ResourceWarning`.

	.. versionchanged:: 3.6
	Add the source parameter.


	.. function:: showwarning(message, category, filename, lineno, file=None, line=None)

	Write a warning to a file. The default implementation calls
	``formatwarning(message, category, filename, lineno, line)`` and writes the
	resulting string to file, which defaults to ``sys.stderr``. You may replace
	this function with any callable by assigning to ``warnings.showwarning``.
	line is a line of source code to be included in the warning
	message; if line is not supplied, :func:`showwarning` will
	try to read the line specified by filename and lineno.


	.. function:: formatwarning(message, category, filename, lineno, line=None)

	Format a warning the standard way. This returns a string which may contain
	embedded newlines and ends in a newline. line is a line of source code to
	be included in the warning message; if line is not supplied,
	:func:`formatwarning` will try to read the line specified by filename and
	lineno.


	.. function:: filterwarnings(action, message='', category=Warning, module='', lineno=0, append=False)

	Insert an entry into the list of :ref:`warnings filter specifications
	<warning-filter>`. The entry is inserted at the front by default; if
	append is true, it is inserted at the end. This checks the types of the
	arguments, compiles the message and module regular expressions, and
	inserts them as a tuple in the list of warnings filters. Entries closer to
	the front of the list override entries later in the list, if both match a
	particular warning. Omitted arguments default to a value that matches
	everything.


	.. function:: simplefilter(action, category=Warning, lineno=0, append=False)

	Insert a simple entry into the list of :ref:`warnings filter specifications
	<warning-filter>`. The meaning of the function parameters is as for
	:func:`filterwarnings`, but regular expressions are not needed as the filter
	inserted always matches any message in any module as long as the category and
	line number match.


	.. function:: resetwarnings()

	Reset the warnings filter. This discards the effect of all previous calls to
	:func:`filterwarnings`, including that of the :option:`-W` command line options
	and calls to :func:`simplefilter`.


	Available Context Managers
	--------------------------

	.. class:: catch_warnings(\*, record=False, module=None)

	A context manager that copies and, upon exit, restores the warnings filter
	and the :func:`showwarning` function.
	If the record argument is :const:`False` (the default) the context manager
	returns :class:`None` on entry. If record is :const:`True`, a list is
	returned that is progressively populated with objects as seen by a custom
	:func:`showwarning` function (which also suppresses output to ``sys.stdout``).
	Each object in the list has attributes with the same names as the arguments to
	:func:`showwarning`.

	The module argument takes a module that will be used instead of the
	module returned when you import :mod:`warnings` whose filter will be
	protected. This argument exists primarily for testing the :mod:`warnings`
	module itself.

	.. note::

	The :class:`catch_warnings` manager works by replacing and
	then later restoring the module's
	:func:`showwarning` function and internal list of filter
	specifications. This means the context manager is modifying
	global state and therefore is not thread-safe.