mirror of
https://github.com/python/cpython.git
synced 2024-11-28 16:45:42 +01:00
9b99747386
- 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
517 lines
23 KiB
ReStructuredText
517 lines
23 KiB
ReStructuredText
: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.
|