You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
430 lines
19 KiB
430 lines
19 KiB
:mod:`warnings` --- Warning control
|
|
===================================
|
|
|
|
.. index:: single: warnings
|
|
|
|
.. module:: warnings
|
|
:synopsis: Issue warning messages and control their disposition.
|
|
|
|
|
|
.. versionadded:: 2.1
|
|
|
|
**Source code:** :source:`Lib/warnings.py`
|
|
|
|
--------------
|
|
|
|
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. 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 (ignored by default). |
|
|
+----------------------------------+-----------------------------------------------+
|
|
| :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 constructs |
|
|
| | that will change semantically in the future. |
|
|
+----------------------------------+-----------------------------------------------+
|
|
| :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. |
|
|
+----------------------------------+-----------------------------------------------+
|
|
|
|
While these are technically built-in 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.
|
|
|
|
.. versionchanged:: 2.7
|
|
:exc:`DeprecationWarning` is ignored by default.
|
|
|
|
|
|
.. _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 match 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 |
|
|
+===============+==============================================+
|
|
| ``"error"`` | turn matching warnings into exceptions |
|
|
+---------------+----------------------------------------------+
|
|
| ``"ignore"`` | never print matching warnings |
|
|
+---------------+----------------------------------------------+
|
|
| ``"always"`` | always print matching warnings |
|
|
+---------------+----------------------------------------------+
|
|
| ``"default"`` | print the first occurrence of matching |
|
|
| | warnings for each location where the warning |
|
|
| | is issued |
|
|
+---------------+----------------------------------------------+
|
|
| ``"module"`` | print the first occurrence of matching |
|
|
| | warnings for each module where the warning |
|
|
| | is issued |
|
|
+---------------+----------------------------------------------+
|
|
| ``"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)``.
|
|
|
|
The warnings filter is initialized by :option:`-W` options passed to the Python
|
|
interpreter command line. The interpreter saves the arguments for all
|
|
:option:`-W` options 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``).
|
|
|
|
|
|
Default Warning Filters
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
By default, Python installs several warning filters, which can be overridden by
|
|
the command-line options passed to :option:`-W` and calls to
|
|
:func:`filterwarnings`.
|
|
|
|
* :exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, and
|
|
:exc:`ImportWarning` are ignored.
|
|
|
|
* :exc:`BytesWarning` is ignored unless the :option:`-b` option is given once or
|
|
twice; in this case this warning is either printed (``-b``) or turned into an
|
|
exception (``-bb``).
|
|
|
|
|
|
.. _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, 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).
|
|
|
|
|
|
Updating Code For New Versions of Python
|
|
----------------------------------------
|
|
|
|
Warnings that are only of interest to the developer are ignored by default. As
|
|
such you should make sure to test your code with typically ignored warnings
|
|
made visible. You can do this from the command-line by passing :option:`-Wd <-W>`
|
|
to the interpreter (this is shorthand for :option:`!-W default`). This enables
|
|
default handling for all warnings, including those that are ignored by default.
|
|
To change what action is taken for encountered warnings you simply 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.
|
|
|
|
To programmatically do the same as :option:`!-Wd`, use::
|
|
|
|
warnings.simplefilter('default')
|
|
|
|
Make sure to execute this code as soon as possible. This prevents the
|
|
registering of what warnings have been raised from unexpectedly influencing how
|
|
future warnings are treated.
|
|
|
|
Having certain warnings ignored by default is done to prevent a user from
|
|
seeing warnings that are only of interest to the developer. As you do not
|
|
necessarily have control over what interpreter a user uses to run their code,
|
|
it is possible that a new version of Python will be released between your
|
|
release cycles. The new interpreter release could trigger new warnings in your
|
|
code that were not there in an older interpreter, e.g.
|
|
:exc:`DeprecationWarning` for a module that you are using. While you as a
|
|
developer want to be notified that your code is using a deprecated module, to a
|
|
user this information is essentially noise and provides no benefit to them.
|
|
|
|
|
|
.. _warning-functions:
|
|
|
|
Available Functions
|
|
-------------------
|
|
|
|
|
|
.. function:: warn(message[, category[, stacklevel]])
|
|
|
|
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).
|
|
|
|
|
|
.. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])
|
|
|
|
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).
|
|
|
|
.. versionchanged:: 2.5
|
|
Added the *module_globals* parameter.
|
|
|
|
|
|
.. function:: warnpy3k(message[, category[, stacklevel]])
|
|
|
|
Issue a warning related to Python 3.x deprecation. Warnings are only shown
|
|
when Python is started with the -3 option. Like :func:`warn` *message* must
|
|
be a string and *category* a subclass of :exc:`Warning`. :func:`warnpy3k`
|
|
is using :exc:`DeprecationWarning` as default warning class.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
|
|
.. function:: showwarning(message, category, filename, lineno[, file[, line]])
|
|
|
|
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 an alternative implementation 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*.
|
|
|
|
.. versionchanged:: 2.7
|
|
The *line* argument is required to be supported.
|
|
|
|
|
|
.. function:: formatwarning(message, category, filename, lineno[, line])
|
|
|
|
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*.
|
|
|
|
.. versionchanged:: 2.6
|
|
Added the *line* argument.
|
|
|
|
|
|
.. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])
|
|
|
|
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[, lineno[, append]]])
|
|
|
|
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.
|
|
|
|
.. note::
|
|
|
|
In Python 3, the arguments to the constructor for
|
|
:class:`catch_warnings` are keyword-only arguments.
|
|
|
|
.. versionadded:: 2.6
|
|
|