28.6. "warnings" — Warning control
**********************************

New in version 2.1.

**Source code:** 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 "warn()" function
defined in this module.  (C programmers use "PyErr_WarnEx()"; see
Exception Handling 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 "filterwarnings()" and
reset to its default state by calling "resetwarnings()".

The printing of warning messages is done by calling "showwarning()",
which may be overridden; the default implementation of this function
formats the message by calling "formatwarning()", which is also
available for use by custom implementations.

See also: "logging.captureWarnings()" allows you to handle all
  warnings with the standard logging infrastructure.


28.6.1. 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:

+------------------------------------+-------------------------------------------------+
| Class                              | Description                                     |
+====================================+=================================================+
| "Warning"                          | This is the base class of all warning category  |
|                                    | classes.  It is a subclass of "Exception".      |
+------------------------------------+-------------------------------------------------+
| "UserWarning"                      | The default category for "warn()".              |
+------------------------------------+-------------------------------------------------+
| "DeprecationWarning"               | Base category for warnings about deprecated     |
|                                    | features (ignored by default).                  |
+------------------------------------+-------------------------------------------------+
| "SyntaxWarning"                    | Base category for warnings about dubious        |
|                                    | syntactic features.                             |
+------------------------------------+-------------------------------------------------+
| "RuntimeWarning"                   | Base category for warnings about dubious        |
|                                    | runtime features.                               |
+------------------------------------+-------------------------------------------------+
| "FutureWarning"                    | Base category for warnings about constructs     |
|                                    | that will change semantically in the future.    |
+------------------------------------+-------------------------------------------------+
| "PendingDeprecationWarning"        | Base category for warnings about features that  |
|                                    | will be deprecated in the future (ignored by    |
|                                    | default).                                       |
+------------------------------------+-------------------------------------------------+
| "ImportWarning"                    | Base category for warnings triggered during the |
|                                    | process of importing a module (ignored by       |
|                                    | default).                                       |
+------------------------------------+-------------------------------------------------+
| "UnicodeWarning"                   | Base category for warnings related to Unicode.  |
+------------------------------------+-------------------------------------------------+
| "BytesWarning"                     | Base category for warnings related to bytes and |
|                                    | bytearray.                                      |
+------------------------------------+-------------------------------------------------+

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 "Warning" class.

Changed in version 2.7: "DeprecationWarning" is ignored by default.


28.6.2. 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 "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 "Warning" class is derived from the built-in "Exception"
class, to turn a warning into an error we simply raise
"category(message)".

The warnings filter is initialized by "-W" options passed to the
Python interpreter command line.  The interpreter saves the arguments
for all "-W" options without interpretation in "sys.warnoptions"; the
"warnings" module parses these when it is first imported (invalid
options are ignored, after printing a message to "sys.stderr").


28.6.2.1. Default Warning Filters
---------------------------------

By default, Python installs several warning filters, which can be
overridden by the command-line options passed to "-W" and calls to
"filterwarnings()".

* "DeprecationWarning" and "PendingDeprecationWarning", and
  "ImportWarning" are ignored.

* "BytesWarning" is ignored unless the "-b" option is given once or
  twice; in this case this warning is either printed ("-b") or turned
  into an exception ("-bb").


28.6.3. 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 "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 "catch_warnings" context manager at the same time, the
behavior is undefined.


28.6.4. Testing Warnings
========================

To test warnings raised by code, use the "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 "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 "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).


28.6.5. 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 "-Wd" to the interpreter (this is shorthand for "-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
"-W", e.g. "-W error". See the "-W" flag for more details on what is
possible.

To programmatically do the same as "-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. "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.


28.6.6. Available Functions
===========================

warnings.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 "UserWarning".  Alternatively *message*
   can be a "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 "deprecation()"’s caller, rather
   than to the source of "deprecation()" itself (since the latter
   would defeat the purpose of the warning message).

warnings.warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]])

   This is a low-level interface to the functionality of "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 "Warning" or *message* may
   be a "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).

   Changed in version 2.5: Added the *module_globals* parameter.

warnings.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 "warn()"
   *message* must be a string and *category* a subclass of "Warning".
   "warnpy3k()" is using "DeprecationWarning" as default warning
   class.

   New in version 2.6.

warnings.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, "showwarning()" will try to read the line
   specified by *filename* and *lineno*.

   Changed in version 2.7: The *line* argument is required to be
   supported.

warnings.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, "formatwarning()" will try to read the line specified
   by *filename* and *lineno*.

   Changed in version 2.6: Added the *line* argument.

warnings.filterwarnings(action[, message[, category[, module[, lineno[, append]]]]])

   Insert an entry into the list of warnings filter specifications.
   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.

warnings.simplefilter(action[, category[, lineno[, append]]])

   Insert a simple entry into the list of warnings filter
   specifications.  The meaning of the function parameters is as for
   "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.

warnings.resetwarnings()

   Reset the warnings filter.  This discards the effect of all
   previous calls to "filterwarnings()", including that of the "-W"
   command line options and calls to "simplefilter()".


28.6.7. Available Context Managers
==================================

class warnings.catch_warnings([*, record=False, module=None])

   A context manager that copies and, upon exit, restores the warnings
   filter and the "showwarning()" function. If the *record* argument
   is "False" (the default) the context manager returns "None" on
   entry. If *record* is "True", a list is returned that is
   progressively populated with objects as seen by a custom
   "showwarning()" function (which also suppresses output to
   "sys.stdout"). Each object in the list has attributes with the same
   names as the arguments to "showwarning()".

   The *module* argument takes a module that will be used instead of
   the module returned when you import "warnings" whose filter will be
   protected. This argument exists primarily for testing the
   "warnings" module itself.

   Note: The "catch_warnings" manager works by replacing and then
     later restoring the module’s "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
     "catch_warnings" are keyword-only arguments.

   New in version 2.6.
