27.5. warnings — Warning control

Python 3.2.1

27.5. warnings — Warning control

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.

27.5.1. 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 buffer.
ResourceWarning Base category for warnings related to resource usage.

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.

27.5.2. 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:




    turn matching warnings into exceptions


    never print matching warnings


    always print matching warnings


    print the first occurrence of matching warnings for each location where the warning is issued


    print the first occurrence of matching warnings for each module where the warning is issued


    print only the first occurrence of matching warnings, regardless of location

  • message is a string containing a regular expression that the warning message must match (the match 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 match 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). 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().

Changed in version 3.2: DeprecationWarning is now ignored by default in addition to PendingDeprecationWarning.

27.5.3. 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():

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.

27.5.4. 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.
    # Trigger a warning.
    # 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).

27.5.5. 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:


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.

The unittest module has been also updated to use the 'default' filter while running tests.

27.5.6. Available Functions

warnings.warn(message, category=None, stacklevel=1)

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=None, registry=None, module_globals=None)

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).

warnings.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 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.

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

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

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=Warning, lineno=0, append=False)

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.


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().

27.5.7. 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.


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.