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:mod:`warnings` --- Warning control
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===================================
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.. index:: single: warnings
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:synopsis: Issue warning messages and control their disposition.
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Warning messages are typically issued in situations where it is useful to alert
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the user of some condition in a program, where that condition (normally) doesn't
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warrant raising an exception and terminating the program. For example, one
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might want to issue a warning when a program uses an obsolete module.
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Python programmers issue warnings by calling the :func:`warn` function defined
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in this module. (C programmers use :cfunc:`PyErr_WarnEx`; see
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:ref:`exceptionhandling` for details).
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Warning messages are normally written to ``sys.stderr``, but their disposition
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can be changed flexibly, from ignoring all warnings to turning them into
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exceptions. The disposition of warnings can vary based on the warning category
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(see below), the text of the warning message, and the source location where it
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is issued. Repetitions of a particular warning for the same source location are
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There are two stages in warning control: first, each time a warning is issued, a
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determination is made whether a message should be issued or not; next, if a
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message is to be issued, it is formatted and printed using a user-settable hook.
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The determination whether to issue a warning message is controlled by the
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warning filter, which is a sequence of matching rules and actions. Rules can be
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added to the filter by calling :func:`filterwarnings` and reset to its default
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state by calling :func:`resetwarnings`.
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The printing of warning messages is done by calling :func:`showwarning`, which
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may be overridden; the default implementation of this function formats the
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message by calling :func:`formatwarning`, which is also available for use by
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custom implementations.
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.. _warning-categories:
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There are a number of built-in exceptions that represent warning categories.
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This categorization is useful to be able to filter out groups of warnings. The
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following warnings category classes are currently defined:
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+----------------------------------+-----------------------------------------------+
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| Class | Description |
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+==================================+===============================================+
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| :exc:`Warning` | This is the base class of all warning |
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| | category classes. It is a subclass of |
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| | :exc:`Exception`. |
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+----------------------------------+-----------------------------------------------+
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| :exc:`UserWarning` | The default category for :func:`warn`. |
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+----------------------------------+-----------------------------------------------+
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| :exc:`DeprecationWarning` | Base category for warnings about deprecated |
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+----------------------------------+-----------------------------------------------+
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| :exc:`SyntaxWarning` | Base category for warnings about dubious |
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| | syntactic features. |
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+----------------------------------+-----------------------------------------------+
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| :exc:`RuntimeWarning` | Base category for warnings about dubious |
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| | runtime features. |
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+----------------------------------+-----------------------------------------------+
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| :exc:`FutureWarning` | Base category for warnings about constructs |
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| | that will change semantically in the future. |
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+----------------------------------+-----------------------------------------------+
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| :exc:`PendingDeprecationWarning` | Base category for warnings about features |
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| | that will be deprecated in the future |
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| | (ignored by default). |
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+----------------------------------+-----------------------------------------------+
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| :exc:`ImportWarning` | Base category for warnings triggered during |
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| | the process of importing a module (ignored by |
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+----------------------------------+-----------------------------------------------+
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| :exc:`UnicodeWarning` | Base category for warnings related to |
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+----------------------------------+-----------------------------------------------+
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While these are technically built-in exceptions, they are documented here,
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because conceptually they belong to the warnings mechanism.
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User code can define additional warning categories by subclassing one of the
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standard warning categories. A warning category must always be a subclass of
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the :exc:`Warning` class.
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The warnings filter controls whether warnings are ignored, displayed, or turned
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into errors (raising an exception).
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Conceptually, the warnings filter maintains an ordered list of filter
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specifications; any specific warning is matched against each filter
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specification in the list in turn until a match is found; the match determines
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the disposition of the match. Each entry is a tuple of the form (*action*,
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*message*, *category*, *module*, *lineno*), where:
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* *action* is one of the following strings:
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+---------------+----------------------------------------------+
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| Value | Disposition |
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+===============+==============================================+
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| ``"error"`` | turn matching warnings into exceptions |
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+---------------+----------------------------------------------+
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| ``"ignore"`` | never print matching warnings |
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+---------------+----------------------------------------------+
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| ``"always"`` | always print matching warnings |
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+---------------+----------------------------------------------+
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| ``"default"`` | print the first occurrence of matching |
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| | warnings for each location where the warning |
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+---------------+----------------------------------------------+
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| ``"module"`` | print the first occurrence of matching |
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| | warnings for each module where the warning |
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+---------------+----------------------------------------------+
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| ``"once"`` | print only the first occurrence of matching |
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| | warnings, regardless of location |
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+---------------+----------------------------------------------+
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* *message* is a string containing a regular expression that the warning message
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must match (the match is compiled to always be case-insensitive)
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* *category* is a class (a subclass of :exc:`Warning`) of which the warning
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category must be a subclass in order to match
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* *module* is a string containing a regular expression that the module name must
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match (the match is compiled to be case-sensitive)
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* *lineno* is an integer that the line number where the warning occurred must
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match, or ``0`` to match all line numbers
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Since the :exc:`Warning` class is derived from the built-in :exc:`Exception`
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class, to turn a warning into an error we simply raise ``category(message)``.
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The warnings filter is initialized by :option:`-W` options passed to the Python
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interpreter command line. The interpreter saves the arguments for all
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:option:`-W` options without interpretation in ``sys.warnoptions``; the
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:mod:`warnings` module parses these when it is first imported (invalid options
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are ignored, after printing a message to ``sys.stderr``).
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The warnings that are ignored by default may be enabled by passing :option:`-Wd`
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to the interpreter. This enables default handling for all warnings, including
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those that are normally ignored by default. This is particular useful for
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enabling ImportWarning when debugging problems importing a developed package.
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ImportWarning can also be enabled explicitly in Python code using::
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warnings.simplefilter('default', ImportWarning)
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.. _warning-suppress:
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Temporarily Suppressing Warnings
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--------------------------------
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If you are using code that you know will raise a warning, such as a deprecated
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function, but do not want to see the warning, then it is possible to suppress
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the warning using the :class:`catch_warnings` context manager::
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warnings.warn("deprecated", DeprecationWarning)
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with warnings.catch_warnings():
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warnings.simplefilter("ignore")
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While within the context manager all warnings will simply be ignored. This
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allows you to use known-deprecated code without having to see the warning while
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not suppressing the warning for other code that might not be aware of its use
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To test warnings raised by code, use the :class:`catch_warnings` context
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manager. With it you can temporarily mutate the warnings filter to facilitate
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your testing. For instance, do the following to capture all raised warnings to
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warnings.warn("deprecated", DeprecationWarning)
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with warnings.catch_warnings(record=True) as w:
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# Cause all warnings to always be triggered.
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warnings.simplefilter("always")
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assert isinstance(w[-1].category, DeprecationWarning)
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assert "deprecated" in str(w[-1].message)
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One can also cause all warnings to be exceptions by using ``error`` instead of
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``always``. One thing to be aware of is that if a warning has already been
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raised because of a ``once``/``default`` rule, then no matter what filters are
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set the warning will not be seen again unless the warnings registry related to
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the warning has been cleared.
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Once the context manager exits, the warnings filter is restored to its state
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when the context was entered. This prevents tests from changing the warnings
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filter in unexpected ways between tests and leading to indeterminate test
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results. The :func:`showwarning` function in the module is also restored to
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When testing multiple operations that raise the same kind of warning, it
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is important to test them in a manner that confirms each operation is raising
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a new warning (e.g. set warnings to be raised as exceptions and check the
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operations raise exceptions, check that the length of the warning list
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continues to increase after each operation, or else delete the previous
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entries from the warnings list before each new operation).
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.. _warning-functions:
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.. function:: warn(message[, category[, stacklevel]])
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Issue a warning, or maybe ignore it or raise an exception. The *category*
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argument, if given, must be a warning category class (see above); it defaults to
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:exc:`UserWarning`. Alternatively *message* can be a :exc:`Warning` instance,
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in which case *category* will be ignored and ``message.__class__`` will be used.
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In this case the message text will be ``str(message)``. This function raises an
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exception if the particular warning issued is changed into an error by the
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warnings filter see above. The *stacklevel* argument can be used by wrapper
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functions written in Python, like this::
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def deprecation(message):
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warnings.warn(message, DeprecationWarning, stacklevel=2)
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This makes the warning refer to :func:`deprecation`'s caller, rather than to the
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source of :func:`deprecation` itself (since the latter would defeat the purpose
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of the warning message).
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.. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])
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This is a low-level interface to the functionality of :func:`warn`, passing in
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explicitly the message, category, filename and line number, and optionally the
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module name and the registry (which should be the ``__warningregistry__``
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dictionary of the module). The module name defaults to the filename with
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``.py`` stripped; if no registry is passed, the warning is never suppressed.
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*message* must be a string and *category* a subclass of :exc:`Warning` or
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*message* may be a :exc:`Warning` instance, in which case *category* will be
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*module_globals*, if supplied, should be the global namespace in use by the code
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for which the warning is issued. (This argument is used to support displaying
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source for modules found in zipfiles or other non-filesystem import
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.. versionchanged:: 2.5
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Added the *module_globals* parameter.
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.. function:: warnpy3k(message[, category[, stacklevel]])
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Issue a warning related to Python 3.x deprecation. Warnings are only shown
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when Python is started with the -3 option. Like :func:`warn` *message* must
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be a string and *category* a subclass of :exc:`Warning`. :func:`warnpy3k`
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is using :exc:`DeprecationWarning` as default warning class.
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.. function:: showwarning(message, category, filename, lineno[, file[, line]])
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Write a warning to a file. The default implementation calls
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``formatwarning(message, category, filename, lineno, line)`` and writes the
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resulting string to *file*, which defaults to ``sys.stderr``. You may replace
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this function with an alternative implementation by assigning to
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``warnings.showwarning``.
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*line* is a line of source code to be included in the warning
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message; if *line* is not supplied, :func:`showwarning` will
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try to read the line specified by *filename* and *lineno*.
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.. versionchanged:: 2.6
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Added the *line* argument. Implementations that lack the new argument
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will trigger a :exc:`DeprecationWarning`.
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.. function:: formatwarning(message, category, filename, lineno[, line])
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Format a warning the standard way. This returns a string which may contain
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embedded newlines and ends in a newline. *line* is
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a line of source code to be included in the warning message; if *line* is not supplied,
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:func:`formatwarning` will try to read the line specified by *filename* and *lineno*.
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.. versionchanged:: 2.6
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Added the *line* argument.
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.. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])
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Insert an entry into the list of warnings filters. The entry is inserted at the
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front by default; if *append* is true, it is inserted at the end. This checks
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the types of the arguments, compiles the message and module regular expressions,
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and inserts them as a tuple in the list of warnings filters. Entries closer to
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the front of the list override entries later in the list, if both match a
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particular warning. Omitted arguments default to a value that matches
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.. function:: simplefilter(action[, category[, lineno[, append]]])
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Insert a simple entry into the list of warnings filters. The meaning of the
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function parameters is as for :func:`filterwarnings`, but regular expressions
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are not needed as the filter inserted always matches any message in any module
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as long as the category and line number match.
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.. function:: resetwarnings()
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Reset the warnings filter. This discards the effect of all previous calls to
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:func:`filterwarnings`, including that of the :option:`-W` command line options
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and calls to :func:`simplefilter`.
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Available Context Managers
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--------------------------
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.. class:: catch_warnings([\*, record=False, module=None])
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A context manager that copies and, upon exit, restores the warnings filter
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and the :func:`showwarning` function.
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If the *record* argument is :const:`False` (the default) the context manager
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returns :class:`None` on entry. If *record* is :const:`True`, a list is
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returned that is progressively populated with objects as seen by a custom
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:func:`showwarning` function (which also suppresses output to ``sys.stdout``).
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Each object in the list has attributes with the same names as the arguments to
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The *module* argument takes a module that will be used instead of the
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module returned when you import :mod:`warnings` whose filter will be
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protected. This argument exists primarily for testing the :mod:`warnings`
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In Python 3.0, the arguments to the constructor for
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:class:`catch_warnings` are keyword-only arguments.
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.. versionadded:: 2.6
added
removed
Doc/library/warnings.rst
