30.5. 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.

See also

logging.captureWarnings() allows you to handle all warnings with the standard logging infrastructure.

30.5.1. 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.

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.

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 when those warnings are intended for other Python developers (ignored by default, unless triggered by code in __main__).
SyntaxWarning Base category for warnings about dubious syntactic features.
RuntimeWarning Base category for warnings about dubious runtime features.
FutureWarning Base category for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
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.
ResourceWarning Base category for warnings related to resource usage.

Changed in version 3.7: Previously DeprecationWarning and 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.

30.5.2. 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 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 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).

If a warning is reported and doesn’t match any registered filter then the “default” action is applied (hence its name). Describing Warning Filters

The warnings filter is initialized by -W options passed to the Python interpreter command line and the PYTHONWARNINGS environment variable. The interpreter saves the arguments for all supplied entries 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).

Individual warnings filters are specified as a sequence of fields separated by colons:


The meaning of each of these fields is as described in The Warnings Filter. When listing multiple filters on a single line (as for 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

By default, Python installs several warning filters, which can be overridden by the -W command-line option, the PYTHONWARNINGS environment variable and calls to filterwarnings().

In regular release builds, the default warning filter has the following entries (in order of precedence):


In debug builds, the list of default warning filters is empty.

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

Changed in version 3.7: DeprecationWarning is once again shown by default when triggered directly by code in __main__.

Changed in version 3.7: BytesWarning no longer appears in the default filter list and is instead configured via sys.warnoptions when -b is specified twice. 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 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

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 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,

30.5.3. 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 (even when warnings have been explicitly configured via the command line), 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.

30.5.4. 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).

30.5.5. 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 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 unittest module does this).

In less ideal cases, applications can be checked for use of deprecated interfaces by passing -Wd to the Python interpreter (this is shorthand for -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 -W (e.g. -W error). See the -W flag for more details on what is possible.

30.5.6. Available Functions

warnings.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 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).

source, if supplied, is the destroyed object which emitted a ResourceWarning.

Changed in version 3.6: Added source parameter.

warnings.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 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).

source, if supplied, is the destroyed object which emitted a ResourceWarning.

Changed in version 3.6: Add the source parameter.

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 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, 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().

30.5.7. 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.