: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_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.

.. _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-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.

.. _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 isinstance(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.

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-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).

   .. versionchanged:: 2.5

      Added the *module_globals* parameter.

.. function:: warnpy3k(message[, category[, stacklevel]])

   Issue a warning related to Python 3.x deprecation. Warnings are only shown 

   when Python is started with the -3 option. Like :func:`warn` *message* must

   be a string and *category* a subclass of :exc:`Warning`. :func:`warnpy3k`

   is using :exc:`DeprecationWarning` as default warning class.

.. function:: showwarning(message, category, filename, lineno[, file[, line]])

   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 an alternative implementation 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*.

   .. versionchanged:: 2.6

      Added the *line* argument. Implementations that lack the new argument

      will trigger a :exc:`DeprecationWarning`.

.. function:: formatwarning(message, category, filename, lineno[, line])

   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*.

   .. versionchanged:: 2.6

      Added the *line* argument.

.. 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`.

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::

        In Python 3.0, the arguments to the constructor for

        :class:`catch_warnings` are keyword-only arguments.

    .. versionadded:: 2.6