:mod:`!warnings` --- Warning control

.. module:: warnings

:synopsis: Issue warning messages and control their disposition.

**Source code:** :source:`Lib/warnings.py`

.. index:: single: warnings

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 :c:func:`PyErr_WarnEx`; see

:ref:`exceptionhandling` for details).

Warning messages are normally written to :data:`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 :ref:`warning category

<warning-categories>`, 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

:ref:`warning filter <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.

.. seealso::

:func:`logging.captureWarnings` allows you to handle all warnings with

the standard logging infrastructure.

.. _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.

While these are technically

:ref:`built-in exceptions <warning-categories-as-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.

The following warnings category classes are currently defined:

.. tabularcolumns:: |l|p{0.6\linewidth}|

| Class | Description |

| :exc:`Warning` | Base class for warning categories. It is a |

| | subclass of :exc:`Exception`. |

| :exc:`UserWarning` | Base class for warnings generated by user |

| | code. The default category for :func:`warn`. |

| :exc:`DeprecationWarning` | Base class for warnings about deprecated |

| | features when those warnings are intended for |

| | other Python developers (ignored by default, |

| | unless triggered by code in ``__main__``). |

| :exc:`PendingDeprecationWarning` | Base class for warnings about features |

| | that will be deprecated in the future |

| | (ignored by default). |

| :exc:`SyntaxWarning` | Base class for warnings about dubious syntax |

| | (typically emitted when compiling Python |

| | source code, and hence may not be suppressed |

| | by runtime filters). |

| :exc:`RuntimeWarning` | Base class for warnings about dubious runtime |

| | behavior. |

| :exc:`FutureWarning` | Base class for warnings about deprecated |

| | features when those warnings are intended for |

| | end users of applications that are written in |

| | Python. |

| :exc:`ImportWarning` | Base class for warnings triggered during |

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

| | default). |

| :exc:`UnicodeWarning` | Base class for warnings related to |

| | Unicode. |

| :exc:`EncodingWarning` | Base class for warnings related to encodings. |

| | See :ref:`io-encoding-warning` for details. |

| :exc:`BytesWarning` | Base class for warnings related to |

| | :class:`bytes` and :class:`bytearray`. |

| :exc:`ResourceWarning` | Base class for warnings related to |

| | resource usage (ignored by default). |

.. versionchanged:: 3.7

Previously :exc:`DeprecationWarning` and :exc:`FutureWarning` were

distinguished based on whether a feature was being removed entirely or

changing its behaviour. They are now distinguished based on their

intended audience and the way they're handled by the default warnings

filters.

.. _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 filter 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 |

+===============+==============================================+

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

| | warnings for each location (module + |

| | line number) where the warning is issued |

+---------------+----------------------------------------------+

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

+---------------+----------------------------------------------+

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

+---------------+----------------------------------------------+

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

+---------------+----------------------------------------------+

| ``"all"`` | alias to "always" |

+---------------+----------------------------------------------+

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

| | warnings for each module where the warning |

| | is issued (regardless of line number) |

+---------------+----------------------------------------------+

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

| | warnings, regardless of location |

+---------------+----------------------------------------------+

* *message* is a string containing a regular expression that the start of

the warning message must match, case-insensitively. In :option:`-W` and

:envvar:`PYTHONWARNINGS`, if *message* starts and ends with a forward slash

(``/``), it specifies a regular expression as above;

otherwise it is a literal string that the start of the

warning message must match (case-insensitively), ignoring any whitespace at

the start or end of *message*.

* *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 start of the

fully qualified module name must match, case-sensitively. In :option:`-W` and

:envvar:`PYTHONWARNINGS`, if *module* starts and ends with a forward slash

(``/``), it specifies a regular expression as above;

otherwise it is a literal string that the

fully qualified module name must be equal to (case-sensitively), ignoring any

whitespace at the start or end of *module*.

* *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)``.

If a warning is reported and doesn't match any registered filter then the

"default" action is applied (hence its name).

.. _repeated-warning-suppression-criteria:

Repeated Warning Suppression Criteria

The filters that suppress repeated warnings apply the following criteria to determine if a warning is considered a repeat:

- ``"default"``: A warning is considered a repeat only if the (*message*, *category*, *module*, *lineno*) are all the same.

- ``"module"``: A warning is considered a repeat if the (*message*, *category*, *module*) are the same, ignoring the line number.

- ``"once"``: A warning is considered a repeat if the (*message*, *category*) are the same, ignoring the module and line number.

.. _describing-warning-filters:

Describing Warning Filters

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

interpreter command line and the :envvar:`PYTHONWARNINGS` environment variable.

The interpreter saves the arguments for all supplied entries without

interpretation in :data:`sys.warnoptions`; the :mod:`!warnings` module parses these

when it is first imported (invalid options are ignored, after printing a

message to :data:`sys.stderr`).

Individual warnings filters are specified as a sequence of fields separated by

colons::

The meaning of each of these fields is as described in :ref:`warning-filter`.

When listing multiple filters on a single line (as for

:envvar:`PYTHONWARNINGS`), the individual filters are separated by commas and

the filters listed later take precedence over those listed before them (as

they're applied left-to-right, and the most recently applied filters take

precedence over earlier ones).

Commonly used warning filters apply to either all warnings, warnings in a

particular category, or warnings raised by particular modules or packages.

Some examples::

.. _default-warning-filter:

Default Warning Filter

By default, Python installs several warning filters, which can be overridden by

the :option:`-W` command-line option, the :envvar:`PYTHONWARNINGS` environment

variable and calls to :func:`filterwarnings`.

In regular release builds, the default warning filter has the following entries

(in order of precedence)::

In a :ref:`debug build <debug-build>`, the list of default warning filters is empty.

.. versionchanged:: 3.2

:exc:`DeprecationWarning` is now ignored by default in addition to

:exc:`PendingDeprecationWarning`.

.. versionchanged:: 3.7

:exc:`DeprecationWarning` is once again shown by default when triggered

directly by code in ``__main__``.

.. versionchanged:: 3.7

:exc:`BytesWarning` no longer appears in the default filter list and is

instead configured via :data:`sys.warnoptions` when :option:`-b` is specified

twice.

.. _warning-disable:

Overriding the default filter

Developers of applications written in Python may wish to hide *all* Python level

warnings from their users by default, and only display them when running tests

or otherwise working on the application. The :data:`sys.warnoptions` attribute

used to pass filter configurations to the interpreter can be used as a marker to

indicate whether or not warnings should be disabled::

Developers of test runners for Python code are advised to instead ensure that

*all* warnings are displayed by default for the code under test, using code

like::

Finally, developers of interactive shells that run user code in a namespace

other than ``__main__`` are advised to ensure that :exc:`DeprecationWarning`

messages are made visible by default, using code like the following (where

``user_ns`` is the module used to execute code entered interactively)::

.. _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 (even when warnings have been

explicitly configured via the command line), then it is possible to suppress

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

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.

.. note::

See :ref:`warning-concurrent-safe` for details on the

concurrency-safety of the :class:`catch_warnings` context manager when

used in programs using multiple threads or async functions.

.. _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::

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.

.. note::

See :ref:`warning-concurrent-safe` for details on the

concurrency-safety of the :class:`catch_warnings` context manager when

used in programs using multiple threads or async functions.

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-ignored:

Updating Code For New Versions of Dependencies

Warning categories that are primarily of interest to Python developers (rather

than end users of applications written in Python) are ignored by default.

Notably, this "ignored by default" list includes :exc:`DeprecationWarning`

(for every module except ``__main__``), which means developers should make sure

to test their code with typically ignored warnings made visible in order to

receive timely notifications of future breaking API changes (whether in the

standard library or third party packages).

In the ideal case, the code will have a suitable test suite, and the test runner

will take care of implicitly enabling all warnings when running tests

(the test runner provided by the :mod:`unittest` module does this).

In less ideal cases, applications can be checked for use of deprecated

interfaces by passing :option:`-Wd <-W>` to the Python interpreter (this is

shorthand for :option:`!-W default`) or setting ``PYTHONWARNINGS=default`` in

the environment. This enables default handling for all warnings, including those

that are ignored by default. To change what action is taken for encountered

warnings you can change what argument is passed to :option:`-W` (e.g.

:option:`!-W error`). See the :option:`-W` flag for more details on what is

possible.

.. _warning-functions:

Available Functions

.. function:: warn(message, category=None, stacklevel=1, source=None, *, skip_file_prefixes=())

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

argument, if given, must be a :ref:`warning category class <warning-categories>`; 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

:ref:`warnings filter <warning-filter>`. The *stacklevel* argument can be used by wrapper

functions written in Python, like this::

This makes the warning refer to ``deprecated_api``'s caller, rather than to

the source of ``deprecated_api`` itself (since the latter would defeat the

purpose of the warning message).

The *skip_file_prefixes* keyword argument can be used to indicate which

stack frames are ignored when counting stack levels. This can be useful when

you want the warning to always appear at call sites outside of a package

when a constant *stacklevel* does not fit all call paths or is otherwise

challenging to maintain. If supplied, it must be a tuple of strings. When

prefixes are supplied, stacklevel is implicitly overridden to be ``max(2,

stacklevel)``. To cause a warning to be attributed to the caller from

outside of the current package you might write::

This makes the warning refer to both the ``example.lower.one_way()`` and

``example.higher.another_way()`` call sites only from calling code living

outside of ``example`` package.

*source*, if supplied, is the destroyed object which emitted a

:exc:`ResourceWarning`.

.. versionchanged:: 3.6

Added *source* parameter.

.. versionchanged:: 3.12

Added *skip_file_prefixes*.

.. function:: warn_explicit(message, category, filename, lineno, module=None, registry=None, module_globals=None, source=None)

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

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

other arguments.

*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*, if supplied, should be the module name.

If no module is passed, the module regular expression in

:ref:`warnings filter <warning-filter>` will be tested against the module

names constructed from the path components starting from all parent

directories (with ``/__init__.py``, ``.py`` and, on Windows, ``.pyw``

stripped) and against the filename with ``.py`` stripped.

For example, when the filename is ``'/path/to/package/module.py'``, it will

be tested against ``'path.to.package.module'``, ``'to.package.module'``

``'package.module'``, ``'module'``, and ``'/path/to/package/module'``.

*registry*, if supplied, should be the ``__warningregistry__`` dictionary

of the module.

If no registry is passed, each warning is treated as the first occurrence,

that is, filter actions ``"default"``, ``"module"`` and ``"once"`` are

handled as ``"always"``.

*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).

*source*, if supplied, is the destroyed object which emitted a

:exc:`ResourceWarning`.

.. versionchanged:: 3.6

Add the *source* parameter.

.. versionchanged:: 3.15

If no module is passed, test the filter regular expression against

module names created from the path, not only the path itself.

.. function:: showwarning(message, category, filename, lineno, file=None, line=None)

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 :data:`sys.stderr`. You may replace

this function with any callable 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*.

.. function:: formatwarning(message, category, filename, lineno, line=None)

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*.

.. function:: filterwarnings(action, message='', category=Warning, module='', lineno=0, append=False)

Insert an entry into the list of :ref:`warnings filter specifications

<warning-filter>`. 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=Warning, lineno=0, append=False)

Insert a simple entry into the list of :ref:`warnings filter specifications

<warning-filter>`. 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`.

.. decorator:: deprecated(message, /, *, category=DeprecationWarning, stacklevel=1)

Decorator to indicate that a class, function or overload is deprecated.

When this decorator is applied to an object,

deprecation warnings may be emitted at runtime when the object is used.

:term:`static type checkers <static type checker>`

will also generate a diagnostic on usage of the deprecated object.

Usage::

The warning specified by *category* will be emitted at runtime

on use of deprecated objects. For functions, that happens on calls;

for classes, on instantiation and on creation of subclasses.

If the *category* is ``None``, no warning is emitted at runtime.

The *stacklevel* determines where the

warning is emitted. If it is ``1`` (the default), the warning

is emitted at the direct caller of the deprecated object; if it

is higher, it is emitted further up the stack.

Static type checker behavior is not affected by the *category*

and *stacklevel* arguments.

The deprecation message passed to the decorator is saved in the

``__deprecated__`` attribute on the decorated object.

If applied to an overload, the decorator

must be after the :deco:`~typing.overload` decorator

for the attribute to exist on the overload as returned by

:func:`typing.get_overloads`.

.. versionadded:: 3.13

See :pep:`702`.

Available Context Managers

.. class:: catch_warnings(*, record=False, module=None, action=None, category=Warning, lineno=0, append=False)

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.stderr``).

Each object in the list is guaranteed to have the following attributes:

- ``message``: the warning message (an instance of :exc:`Warning`)

- ``category``: the warning category (a subclass of :exc:`Warning`)

- ``filename``: the file name where the warning occurred (:class:`str`)

- ``lineno``: the line number in the file (:class:`int`)

- ``file``: the file object used for output (if any), or ``None``

- ``line``: the line of source code (if available), or ``None``

- ``source``: the original object that generated the warning (if

available), or ``None``

- ``module``: the module name where the warning occurred

(:class:`str`), or ``None``

.. versionchanged:: 3.6

The ``source`` attribute was added.

.. versionchanged:: 3.15

The ``module`` attribute was added.

The type of these objects is not specified and may change; only the

presence of these attributes is guaranteed.

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.

If the *action* argument is not ``None``, the remaining arguments are

passed to :func:`simplefilter` as if it were called immediately on

entering the context.

See :ref:`warning-filter` for the meaning of the *category* and *lineno*

parameters.

.. note::

See :ref:`warning-concurrent-safe` for details on the

concurrency-safety of the :class:`catch_warnings` context manager when

used in programs using multiple threads or async functions.

.. versionchanged:: 3.11

Added the *action*, *category*, *lineno*, and *append* parameters.

.. _warning-concurrent-safe:

Concurrent safety of Context Managers

The behavior of :class:`catch_warnings` context manager depends on the

:data:`sys.flags.context_aware_warnings` flag. If the flag is true, the

context manager behaves in a concurrent-safe fashion and otherwise not.

Concurrent-safe means that it is both thread-safe and safe to use within

:ref:`asyncio coroutines <coroutine>` and tasks. Being thread-safe means

that behavior is predictable in a multi-threaded program. The flag defaults

to true for free-threaded builds and false otherwise.

If the :data:`~sys.flags.context_aware_warnings` flag is false, then

:class:`catch_warnings` will modify the global attributes of the

:mod:`!warnings` module. This is not safe if used within a concurrent program

(using multiple threads or using asyncio coroutines). For example, if two

or more threads use the :class:`catch_warnings` class at the same time, the

behavior is undefined.

If the flag is true, :class:`catch_warnings` will not modify global

attributes and will instead use a :class:`~contextvars.ContextVar` to

store the newly established warning filtering state. A context variable

provides thread-local storage and it makes the use of :class:`catch_warnings`

thread-safe.

The *record* parameter of the context handler also behaves differently

depending on the value of the flag. When *record* is true and the flag is

false, the context manager works by replacing and then later restoring the

module's :func:`showwarning` function. That is not concurrent-safe.

When *record* is true and the flag is true, the :func:`showwarning` function

is not replaced. Instead, the recording status is indicated by an internal

property in the context variable. In this case, the :func:`showwarning`

function will not be restored when exiting the context handler.

The :data:`~sys.flags.context_aware_warnings` flag can be set the :option:`-X

context_aware_warnings<-X>` command-line option or by the

:envvar:`PYTHON_CONTEXT_AWARE_WARNINGS` environment variable.

.. note::

It is likely that most programs that desire thread-safe

behaviour of the warnings module will also want to set the

:data:`~sys.flags.thread_inherit_context` flag to true. That flag

causes threads created by :class:`threading.Thread` to start

with a copy of the context variables from the thread starting

it. When true, the context established by :class:`catch_warnings`

in one thread will also apply to new threads started by it. If false,

new threads will start with an empty warnings context variable,

meaning that any filtering that was established by a

:class:`catch_warnings` context manager will no longer be active.

.. versionchanged:: 3.14

Added the :data:`sys.flags.context_aware_warnings` flag and the use of a

context variable for :class:`catch_warnings` if the flag is true. Previous

versions of Python acted as if the flag was always set to false.