1 

     2 :mod:`warnings` --- Warning control

     3 ===================================

     4 

     5 .. index:: single: warnings

     6 

     7 .. module:: warnings

     8    :synopsis: Issue warning messages and control their disposition.

     9 

    10 

    11 .. versionadded:: 2.1

    12 

    13 Warning messages are typically issued in situations where it is useful to alert

    14 the user of some condition in a program, where that condition (normally) doesn't

    15 warrant raising an exception and terminating the program.  For example, one

    16 might want to issue a warning when a program uses an obsolete module.

    17 

    18 Python programmers issue warnings by calling the :func:`warn` function defined

    19 in this module.  (C programmers use :cfunc:`PyErr_WarnEx`; see

    20 :ref:`exceptionhandling` for details).

    21 

    22 Warning messages are normally written to ``sys.stderr``, but their disposition

    23 can be changed flexibly, from ignoring all warnings to turning them into

    24 exceptions.  The disposition of warnings can vary based on the warning category

    25 (see below), the text of the warning message, and the source location where it

    26 is issued.  Repetitions of a particular warning for the same source location are

    27 typically suppressed.

    28 

    29 There are two stages in warning control: first, each time a warning is issued, a

    30 determination is made whether a message should be issued or not; next, if a

    31 message is to be issued, it is formatted and printed using a user-settable hook.

    32 

    33 The determination whether to issue a warning message is controlled by the

    34 warning filter, which is a sequence of matching rules and actions. Rules can be

    35 added to the filter by calling :func:`filterwarnings` and reset to its default

    36 state by calling :func:`resetwarnings`.

    37 

    38 The printing of warning messages is done by calling :func:`showwarning`, which

    39 may be overridden; the default implementation of this function formats the

    40 message by calling :func:`formatwarning`, which is also available for use by

    41 custom implementations.

    42 

    43 

    44 .. _warning-categories:

    45 

    46 Warning Categories

    47 ------------------

    48 

    49 There are a number of built-in exceptions that represent warning categories.

    50 This categorization is useful to be able to filter out groups of warnings.  The

    51 following warnings category classes are currently defined:

    52 

    53 +----------------------------------+-----------------------------------------------+

    54 | Class                            | Description                                   |

    55 +==================================+===============================================+

    56 | :exc:`Warning`                   | This is the base class of all warning         |

    57 |                                  | category classes.  It is a subclass of        |

    58 |                                  | :exc:`Exception`.                             |

    59 +----------------------------------+-----------------------------------------------+

    60 | :exc:`UserWarning`               | The default category for :func:`warn`.        |

    61 +----------------------------------+-----------------------------------------------+

    62 | :exc:`DeprecationWarning`        | Base category for warnings about deprecated   |

    63 |                                  | features.                                     |

    64 +----------------------------------+-----------------------------------------------+

    65 | :exc:`SyntaxWarning`             | Base category for warnings about dubious      |

    66 |                                  | syntactic features.                           |

    67 +----------------------------------+-----------------------------------------------+

    68 | :exc:`RuntimeWarning`            | Base category for warnings about dubious      |

    69 |                                  | runtime features.                             |

    70 +----------------------------------+-----------------------------------------------+

    71 | :exc:`FutureWarning`             | Base category for warnings about constructs   |

    72 |                                  | that will change semantically in the future.  |

    73 +----------------------------------+-----------------------------------------------+

    74 | :exc:`PendingDeprecationWarning` | Base category for warnings about features     |

    75 |                                  | that will be deprecated in the future         |

    76 |                                  | (ignored by default).                         |

    77 +----------------------------------+-----------------------------------------------+

    78 | :exc:`ImportWarning`             | Base category for warnings triggered during   |

    79 |                                  | the process of importing a module (ignored by |

    80 |                                  | default).                                     |

    81 +----------------------------------+-----------------------------------------------+

    82 | :exc:`UnicodeWarning`            | Base category for warnings related to         |

    83 |                                  | Unicode.                                      |

    84 +----------------------------------+-----------------------------------------------+

    85 

    86 While these are technically built-in exceptions, they are documented here,

    87 because conceptually they belong to the warnings mechanism.

    88 

    89 User code can define additional warning categories by subclassing one of the

    90 standard warning categories.  A warning category must always be a subclass of

    91 the :exc:`Warning` class.

    92 

    93 

    94 .. _warning-filter:

    95 

    96 The Warnings Filter

    97 -------------------

    98 

    99 The warnings filter controls whether warnings are ignored, displayed, or turned

   100 into errors (raising an exception).

   101 

   102 Conceptually, the warnings filter maintains an ordered list of filter

   103 specifications; any specific warning is matched against each filter

   104 specification in the list in turn until a match is found; the match determines

   105 the disposition of the match.  Each entry is a tuple of the form (*action*,

   106 *message*, *category*, *module*, *lineno*), where:

   107 

   108 * *action* is one of the following strings:

   109 

   110   +---------------+----------------------------------------------+

   111   | Value         | Disposition                                  |

   112   +===============+==============================================+

   113   | ``"error"``   | turn matching warnings into exceptions       |

   114   +---------------+----------------------------------------------+

   115   | ``"ignore"``  | never print matching warnings                |

   116   +---------------+----------------------------------------------+

   117   | ``"always"``  | always print matching warnings               |

   118   +---------------+----------------------------------------------+

   119   | ``"default"`` | print the first occurrence of matching       |

   120   |               | warnings for each location where the warning |

   121   |               | is issued                                    |

   122   +---------------+----------------------------------------------+

   123   | ``"module"``  | print the first occurrence of matching       |

   124   |               | warnings for each module where the warning   |

   125   |               | is issued                                    |

   126   +---------------+----------------------------------------------+

   127   | ``"once"``    | print only the first occurrence of matching  |

   128   |               | warnings, regardless of location             |

   129   +---------------+----------------------------------------------+

   130 

   131 * *message* is a string containing a regular expression that the warning message

   132   must match (the match is compiled to always be  case-insensitive)

   133 

   134 * *category* is a class (a subclass of :exc:`Warning`) of which the warning

   135   category must be a subclass in order to match

   136 

   137 * *module* is a string containing a regular expression that the module name must

   138   match (the match is compiled to be case-sensitive)

   139 

   140 * *lineno* is an integer that the line number where the warning occurred must

   141   match, or ``0`` to match all line numbers

   142 

   143 Since the :exc:`Warning` class is derived from the built-in :exc:`Exception`

   144 class, to turn a warning into an error we simply raise ``category(message)``.

   145 

   146 The warnings filter is initialized by :option:`-W` options passed to the Python

   147 interpreter command line.  The interpreter saves the arguments for all

   148 :option:`-W` options without interpretation in ``sys.warnoptions``; the

   149 :mod:`warnings` module parses these when it is first imported (invalid options

   150 are ignored, after printing a message to ``sys.stderr``).

   151 

   152 The warnings that are ignored by default may be enabled by passing :option:`-Wd`

   153 to the interpreter. This enables default handling for all warnings, including

   154 those that are normally ignored by default. This is particular useful for

   155 enabling ImportWarning when debugging problems importing a developed package.

   156 ImportWarning can also be enabled explicitly in Python code using::

   157 

   158    warnings.simplefilter('default', ImportWarning)

   159 

   160 

   161 .. _warning-suppress:

   162 

   163 Temporarily Suppressing Warnings

   164 --------------------------------

   165 

   166 If you are using code that you know will raise a warning, such as a deprecated

   167 function, but do not want to see the warning, then it is possible to suppress

   168 the warning using the :class:`catch_warnings` context manager::

   169 

   170     import warnings

   171 

   172     def fxn():

   173         warnings.warn("deprecated", DeprecationWarning)

   174 

   175     with warnings.catch_warnings():

   176         warnings.simplefilter("ignore")

   177         fxn()

   178 

   179 While within the context manager all warnings will simply be ignored. This

   180 allows you to use known-deprecated code without having to see the warning while

   181 not suppressing the warning for other code that might not be aware of its use

   182 of deprecated code.

   183 

   184 

   185 .. _warning-testing:

   186 

   187 Testing Warnings

   188 ----------------

   189 

   190 To test warnings raised by code, use the :class:`catch_warnings` context

   191 manager. With it you can temporarily mutate the warnings filter to facilitate

   192 your testing. For instance, do the following to capture all raised warnings to

   193 check::

   194 

   195     import warnings

   196 

   197     def fxn():

   198         warnings.warn("deprecated", DeprecationWarning)

   199 

   200     with warnings.catch_warnings(record=True) as w:

   201         # Cause all warnings to always be triggered.

   202         warnings.simplefilter("always")

   203         # Trigger a warning.

   204         fxn()

   205         # Verify some things

   206         assert len(w) == 1

   207         assert isinstance(w[-1].category, DeprecationWarning)

   208         assert "deprecated" in str(w[-1].message)

   209 

   210 One can also cause all warnings to be exceptions by using ``error`` instead of

   211 ``always``. One thing to be aware of is that if a warning has already been

   212 raised because of a ``once``/``default`` rule, then no matter what filters are

   213 set the warning will not be seen again unless the warnings registry related to

   214 the warning has been cleared.

   215 

   216 Once the context manager exits, the warnings filter is restored to its state

   217 when the context was entered. This prevents tests from changing the warnings

   218 filter in unexpected ways between tests and leading to indeterminate test

   219 results. The :func:`showwarning` function in the module is also restored to

   220 its original value.

   221 

   222 When testing multiple operations that raise the same kind of warning, it

   223 is important to test them in a manner that confirms each operation is raising

   224 a new warning (e.g. set warnings to be raised as exceptions and check the

   225 operations raise exceptions, check that the length of the warning list

   226 continues to increase after each operation, or else delete the previous

   227 entries from the warnings list before each new operation).

   228 

   229 

   230 .. _warning-functions:

   231 

   232 Available Functions

   233 -------------------

   234 

   235 

   236 .. function:: warn(message[, category[, stacklevel]])

   237 

   238    Issue a warning, or maybe ignore it or raise an exception.  The *category*

   239    argument, if given, must be a warning category class (see above); it defaults to

   240    :exc:`UserWarning`.  Alternatively *message* can be a :exc:`Warning` instance,

   241    in which case *category* will be ignored and ``message.__class__`` will be used.

   242    In this case the message text will be ``str(message)``. This function raises an

   243    exception if the particular warning issued is changed into an error by the

   244    warnings filter see above.  The *stacklevel* argument can be used by wrapper

   245    functions written in Python, like this::

   246 

   247       def deprecation(message):

   248           warnings.warn(message, DeprecationWarning, stacklevel=2)

   249 

   250    This makes the warning refer to :func:`deprecation`'s caller, rather than to the

   251    source of :func:`deprecation` itself (since the latter would defeat the purpose

   252    of the warning message).

   253 

   254 

   255 .. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])

   256 

   257    This is a low-level interface to the functionality of :func:`warn`, passing in

   258    explicitly the message, category, filename and line number, and optionally the

   259    module name and the registry (which should be the ``__warningregistry__``

   260    dictionary of the module).  The module name defaults to the filename with

   261    ``.py`` stripped; if no registry is passed, the warning is never suppressed.

   262    *message* must be a string and *category* a subclass of :exc:`Warning` or

   263    *message* may be a :exc:`Warning` instance, in which case *category* will be

   264    ignored.

   265 

   266    *module_globals*, if supplied, should be the global namespace in use by the code

   267    for which the warning is issued.  (This argument is used to support displaying

   268    source for modules found in zipfiles or other non-filesystem import

   269    sources).

   270 

   271    .. versionchanged:: 2.5

   272       Added the *module_globals* parameter.

   273 

   274 

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

   276 

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

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

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

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

   281 

   282 

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

   284 

   285    Write a warning to a file.  The default implementation calls

   286    ``formatwarning(message, category, filename, lineno, line)`` and writes the

   287    resulting string to *file*, which defaults to ``sys.stderr``.  You may replace

   288    this function with an alternative implementation by assigning to

   289    ``warnings.showwarning``.

   290    *line* is a line of source code to be included in the warning

   291    message; if *line* is not supplied, :func:`showwarning` will 

   292    try to read the line specified by *filename* and *lineno*.

   293 

   294    .. versionchanged:: 2.6

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

   296       will trigger a :exc:`DeprecationWarning`.

   297 

   298 

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

   300 

   301    Format a warning the standard way.  This returns a string  which may contain

   302    embedded newlines and ends in a newline.  *line* is 

   303    a line of source code to be included in the warning message; if *line* is not supplied, 

   304    :func:`formatwarning` will try to read the line specified by *filename* and *lineno*.

   305 

   306    .. versionchanged:: 2.6

   307       Added the *line* argument.

   308 

   309 

   310 .. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])

   311 

   312    Insert an entry into the list of warnings filters.  The entry is inserted at the

   313    front by default; if *append* is true, it is inserted at the end. This checks

   314    the types of the arguments, compiles the message and module regular expressions,

   315    and inserts them as a tuple in the  list of warnings filters.  Entries closer to

   316    the front of the list override entries later in the list, if both match a

   317    particular warning.  Omitted arguments default to a value that matches

   318    everything.

   319 

   320 

   321 .. function:: simplefilter(action[, category[, lineno[, append]]])

   322 

   323    Insert a simple entry into the list of warnings filters. The meaning of the

   324    function parameters is as for :func:`filterwarnings`, but regular expressions

   325    are not needed as the filter inserted always matches any message in any module

   326    as long as the category and line number match.

   327 

   328 

   329 .. function:: resetwarnings()

   330 

   331    Reset the warnings filter.  This discards the effect of all previous calls to

   332    :func:`filterwarnings`, including that of the :option:`-W` command line options

   333    and calls to :func:`simplefilter`.

   334 

   335 

   336 Available Context Managers

   337 --------------------------

   338 

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

   340 

   341     A context manager that copies and, upon exit, restores the warnings filter

   342     and the :func:`showwarning` function.

   343     If the *record* argument is :const:`False` (the default) the context manager

   344     returns :class:`None` on entry. If *record* is :const:`True`, a list is

   345     returned that is progressively populated with objects as seen by a custom

   346     :func:`showwarning` function (which also suppresses output to ``sys.stdout``).

   347     Each object in the list has attributes with the same names as the arguments to

   348     :func:`showwarning`.

   349 

   350     The *module* argument takes a module that will be used instead of the

   351     module returned when you import :mod:`warnings` whose filter will be

   352     protected. This argument exists primarily for testing the :mod:`warnings`

   353     module itself.

   354 

   355     .. note::

   356 

   357         In Python 3.0, the arguments to the constructor for

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

   359 

   360     .. versionadded:: 2.6

   361