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cpython/Doc/library/warnings.rst
Nick Coghlan 9b99747386
bpo-31975 (PEP 565): Show DeprecationWarning in __main__ (GH-4458) 
- primary change is to add a new default filter entry for
  'default::DeprecationWarning:__main__'
- secondary change is an internal one to cope with plain
  strings in the warning module's internal filter list
  (this avoids the need to create a compiled regex object
  early on during interpreter startup)
- assorted documentation updates, including many more
  examples of configuring the warnings settings
- additional tests to ensure that both the pure Python and
  the C accelerated warnings modules have the expected
  default configuration
2018-01-08 12:45:02 +10:00

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: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 ``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.
.. 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` | 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 when those warnings are intended for |
| | other Python developers (ignored by default, |
| | unless triggered by code in ``__main__``). |
+----------------------------------+-----------------------------------------------+
| :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 deprecated |
| | features when those warnings are intended for |
| | end users of applications that are written in |
| | Python. |
+----------------------------------+-----------------------------------------------+
| :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. |
+----------------------------------+-----------------------------------------------+
| :exc:`BytesWarning` | Base category for warnings related to |
| | :class:`bytes` and :class:`bytearray`. |
+----------------------------------+-----------------------------------------------+
| :exc:`ResourceWarning` | Base category for warnings related to |
| | resource usage. |
+----------------------------------+-----------------------------------------------+
.. 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 |
+---------------+----------------------------------------------+
| ``"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. The expression 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 expression 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)``.
If a warning is reported and doesn't match any registered filter then the
"default" action is applied (hence its name).
.. _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 ``sys.warnoptions``; the :mod:`warnings` module parses these
when it is first imported (invalid options are ignored, after printing a
message to ``sys.stderr``).
Individual warnings filters are specified as a sequence of fields separated by
colons::
action:message:category:module:line
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 # Show all warnings (even those ignored by default)
ignore # Ignore all warnings
error # Convert all warnings to errors
error::ResourceWarning # Treat ResourceWarning messages as errors
default::DeprecationWarning # Show DeprecationWarning messages
ignore,default:::mymodule # Only report warnings triggered by "mymodule"
error:::mymodule[.*] # Convert warnings to errors in "mymodule"
# and any subpackages of "mymodule"
.. _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)::
default::DeprecationWarning:__main__
ignore::DeprecationWarning
ignore::PendingDeprecationWarning
ignore::ImportWarning
ignore::ResourceWarning
In debug builds, 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::
import sys
if not sys.warnoptions:
import warnings
warnings.simplefilter("ignore")
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::
import sys
if not sys.warnoptions:
import os, warnings
warnings.simplefilter("default") # Change the filter in this process
os.environ["PYTHONWARNINGS"] = "default" # Also affect subprocesses
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)::
import warnings
warnings.filterwarnings("default", category=DeprecationWarning,
module=user_ns.get("__name__"))
.. _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::
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. Note: this can only be guaranteed in a single-threaded
application. If two or more threads use the :class:`catch_warnings` context
manager at the same time, the behavior is undefined.
.. _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 issubclass(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. Note: this can only be guaranteed in a single-threaded
application. If two or more threads use the :class:`catch_warnings` context
manager at the same time, the behavior is undefined.
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)
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).
*source*, if supplied, is the destroyed object which emitted a
:exc:`ResourceWarning`.
.. versionchanged:: 3.6
Added *source* parameter.
.. 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 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).
*source*, if supplied, is the destroyed object which emitted a
:exc:`ResourceWarning`.
.. versionchanged:: 3.6
Add the *source* parameter.
.. 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 ``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`.
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::
The :class:`catch_warnings` manager works by replacing and
then later restoring the module's
:func:`showwarning` function and internal list of filter
specifications. This means the context manager is modifying
global state and therefore is not thread-safe.
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