2 :mod:`warnings` --- Warning control
3 ===================================
5 .. index:: single: warnings
8 :synopsis: Issue warning messages and control their disposition.
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.
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).
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
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.
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`.
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.
44 .. _warning-categories:
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:
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 |
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 |
81 +----------------------------------+-----------------------------------------------+
82 | :exc:`UnicodeWarning` | Base category for warnings related to |
84 +----------------------------------+-----------------------------------------------+
86 While these are technically built-in exceptions, they are documented here,
87 because conceptually they belong to the warnings mechanism.
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.
99 The warnings filter controls whether warnings are ignored, displayed, or turned
100 into errors (raising an exception).
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:
108 * *action* is one of the following strings:
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 |
122 +---------------+----------------------------------------------+
123 | ``"module"`` | print the first occurrence of matching |
124 | | warnings for each module where the warning |
126 +---------------+----------------------------------------------+
127 | ``"once"`` | print only the first occurrence of matching |
128 | | warnings, regardless of location |
129 +---------------+----------------------------------------------+
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)
134 * *category* is a class (a subclass of :exc:`Warning`) of which the warning
135 category must be a subclass in order to match
137 * *module* is a string containing a regular expression that the module name must
138 match (the match is compiled to be case-sensitive)
140 * *lineno* is an integer that the line number where the warning occurred must
141 match, or ``0`` to match all line numbers
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)``.
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``).
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::
158 warnings.simplefilter('default', ImportWarning)
161 .. _warning-suppress:
163 Temporarily Suppressing Warnings
164 --------------------------------
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::
173 warnings.warn("deprecated", DeprecationWarning)
175 with warnings.catch_warnings():
176 warnings.simplefilter("ignore")
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
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
198 warnings.warn("deprecated", DeprecationWarning)
200 with warnings.catch_warnings(record=True) as w:
201 # Cause all warnings to always be triggered.
202 warnings.simplefilter("always")
207 assert isinstance(w[-1].category, DeprecationWarning)
208 assert "deprecated" in str(w[-1].message)
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.
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
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).
230 .. _warning-functions:
236 .. function:: warn(message[, category[, stacklevel]])
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::
247 def deprecation(message):
248 warnings.warn(message, DeprecationWarning, stacklevel=2)
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).
255 .. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])
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
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
271 .. versionchanged:: 2.5
272 Added the *module_globals* parameter.
275 .. function:: warnpy3k(message[, category[, stacklevel]])
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.
283 .. function:: showwarning(message, category, filename, lineno[, file[, line]])
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*.
294 .. versionchanged:: 2.6
295 Added the *line* argument. Implementations that lack the new argument
296 will trigger a :exc:`DeprecationWarning`.
299 .. function:: formatwarning(message, category, filename, lineno[, line])
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*.
306 .. versionchanged:: 2.6
307 Added the *line* argument.
310 .. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])
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
321 .. function:: simplefilter(action[, category[, lineno[, append]]])
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.
329 .. function:: resetwarnings()
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`.
336 Available Context Managers
337 --------------------------
339 .. class:: catch_warnings([\*, record=False, module=None])
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
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`
357 In Python 3.0, the arguments to the constructor for
358 :class:`catch_warnings` are keyword-only arguments.
360 .. versionadded:: 2.6