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+
+:mod:`warnings` --- Warning control
+===================================
+
+.. index:: single: warnings
+
+.. module:: warnings
+ :synopsis: Issue warning messages and control their disposition.
+
+
+.. versionadded:: 2.1
+
+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 :cfunc:`PyErr_Warn`; 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.
+
+
+.. _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:
+
++----------------------------------+-----------------------------------------------+
+| 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. |
++----------------------------------+-----------------------------------------------+
+| :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. |
++----------------------------------+-----------------------------------------------+
+
+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 warning message
+ must match (the match 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 match 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``).
+
+The warnings that are ignored by default may be enabled by passing :option:`-Wd`
+to the interpreter. This enables default handling for all warnings, including
+those that are normally ignored by default. This is particular useful for
+enabling ImportWarning when debugging problems importing a developed package.
+ImportWarning can also be enabled explicitly in Python code using::
+
+ warnings.simplefilter('default', ImportWarning)
+
+
+.. _warning-functions:
+
+Available Functions
+-------------------
+
+
+.. function:: warn(message[, category[, stacklevel]])
+
+ 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).
+
+
+.. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])
+
+ 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, and
+ was added in Python 2.5.)
+
+
+.. function:: showwarning(message, category, filename, lineno[, file])
+
+ Write a warning to a file. The default implementation calls
+ ``formatwarning(message, category, filename, lineno)`` and writes the resulting
+ string to *file*, which defaults to ``sys.stderr``. You may replace this
+ function with an alternative implementation by assigning to
+ ``warnings.showwarning``.
+
+
+.. function:: formatwarning(message, category, filename, lineno)
+
+ Format a warning the standard way. This returns a string which may contain
+ embedded newlines and ends in a newline.
+
+
+.. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])
+
+ Insert an entry into the list of warnings filters. 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[, lineno[, append]]])
+
+ Insert a simple entry into the list of warnings filters. 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`.
+