Warnings Capture

Starting from version 3.1, pytest now automatically catches warnings during test execution and displays them at the end of the session:

# content of test_show_warnings.py
import warnings

def api_v1():
    warnings.warn(UserWarning("api v1, should use functions from v2"))
    return 1

def test_one():
    assert api_v1() == 1

Running pytest now produces this output:

$ pytest test_show_warnings.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
collected 1 item

test_show_warnings.py .                                              [100%]

============================= warnings summary =============================
  $REGENDOC_TMPDIR/test_show_warnings.py:5: UserWarning: api v1, should use functions from v2
    warnings.warn(UserWarning("api v1, should use functions from v2"))

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======================= 1 passed, 1 warning in 0.12s =======================

The -W flag can be passed to control which warnings will be displayed or even turn them into errors:

$ pytest -q test_show_warnings.py -W error::UserWarning
F                                                                    [100%]
================================= FAILURES =================================
_________________________________ test_one _________________________________

    def test_one():
>       assert api_v1() == 1

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def api_v1():
>       warnings.warn(UserWarning("api v1, should use functions from v2"))
E       UserWarning: api v1, should use functions from v2

test_show_warnings.py:5: UserWarning
1 failed in 0.12s

The same option can be set in the pytest.ini file using the filterwarnings ini option. For example, the configuration below will ignore all user warnings, but will transform all other warnings into errors.

filterwarnings =

When a warning matches more than one option in the list, the action for the last matching option is performed.

Both -W command-line option and filterwarnings ini option are based on Python’s own -W option and warnings.simplefilter, so please refer to those sections in the Python documentation for other examples and advanced usage.


You can use the @pytest.mark.filterwarnings to add warning filters to specific test items, allowing you to have finer control of which warnings should be captured at test, class or even module level:

import warnings

def api_v1():
    warnings.warn(UserWarning("api v1, should use functions from v2"))
    return 1

@pytest.mark.filterwarnings("ignore:api v1")
def test_one():
    assert api_v1() == 1

Filters applied using a mark take precedence over filters passed on the command line or configured by the filterwarnings ini option.

You may apply a filter to all tests of a class by using the filterwarnings mark as a class decorator or to all tests in a module by setting the pytestmark variable:

# turns all warnings into errors for this module
pytestmark = pytest.mark.filterwarnings("error")

Credits go to Florian Schulze for the reference implementation in the pytest-warnings plugin.

Disabling warnings summary

Although not recommended, you can use the --disable-warnings command-line option to suppress the warning summary entirely from the test run output.

Disabling warning capture entirely

This plugin is enabled by default but can be disabled entirely in your pytest.ini file with:

addopts = -p no:warnings

Or passing -p no:warnings in the command-line. This might be useful if your test suites handles warnings using an external system.

DeprecationWarning and PendingDeprecationWarning

By default pytest will display DeprecationWarning and PendingDeprecationWarning warnings from user code and third-party libraries, as recommended by PEP-0565. This helps users keep their code modern and avoid breakages when deprecated warnings are effectively removed.

Sometimes it is useful to hide some specific deprecation warnings that happen in code that you have no control over (such as third-party libraries), in which case you might use the warning filters options (ini or marks) to ignore those warnings.

For example:

filterwarnings =
    ignore:.*U.*mode is deprecated:DeprecationWarning

This will ignore all warnings of type DeprecationWarning where the start of the message matches the regular expression ".*U.*mode is deprecated".


If warnings are configured at the interpreter level, using the PYTHONWARNINGS environment variable or the -W command-line option, pytest will not configure any filters by default.

Also pytest doesn’t follow PEP-0506 suggestion of resetting all warning filters because it might break test suites that configure warning filters themselves by calling warnings.simplefilter (see issue #2430 for an example of that).

Ensuring code triggers a deprecation warning

You can also use pytest.deprecated_call() for checking that a certain function call triggers a DeprecationWarning or PendingDeprecationWarning:

import pytest

def test_myfunction_deprecated():
    with pytest.deprecated_call():

This test will fail if myfunction does not issue a deprecation warning when called with a 17 argument.

By default, DeprecationWarning and PendingDeprecationWarning will not be caught when using pytest.warns() or recwarn because the default Python warnings filters hide them. If you wish to record them in your own code, use warnings.simplefilter('always'):

import warnings
import pytest

def test_deprecation(recwarn):
    assert len(recwarn) == 1
    assert recwarn.pop(DeprecationWarning)

The recwarn fixture automatically ensures to reset the warnings filter at the end of the test, so no global state is leaked.

Asserting warnings with the warns function

You can check that code raises a particular warning using pytest.warns, which works similar to raises:

import warnings
import pytest

def test_warning():
    with pytest.warns(UserWarning):
        warnings.warn("my warning", UserWarning)

The test will fail if the warning in question is not raised. The keyword argument match can be used to assert that the warning message matches the given regular expression (using re.search()):

>>> with warns(UserWarning, match='must be 0 or None'):
...     warnings.warn("value must be 0 or None", UserWarning)

>>> with warns(UserWarning, match=r'must be \d+$'):
...     warnings.warn("value must be 42", UserWarning)

>>> with warns(UserWarning, match=r'must be \d+$'):
...     warnings.warn("this is not here", UserWarning)
Traceback (most recent call last):
_pytest.outcomes.Failed: DID NOT WARN. No warnings of type ...UserWarning... was emitted...

You can also call pytest.warns on a function or code string:

pytest.warns(expected_warning, func, *args, **kwargs)
pytest.warns(expected_warning, "func(*args, **kwargs)")

The function also returns a list of all raised warnings (as warnings.WarningMessage objects), which you can query for additional information:

with pytest.warns(RuntimeWarning) as record:
    warnings.warn("another warning", RuntimeWarning)

# check that only one warning was raised
assert len(record) == 1
# check that the message matches
assert record[0].message.args[0] == "another warning"

Alternatively, you can examine raised warnings in detail using the recwarn fixture (see below).


DeprecationWarning and PendingDeprecationWarning are treated differently; see Ensuring code triggers a deprecation warning.

Recording warnings

You can record raised warnings either using pytest.warns or with the recwarn fixture.

To record with pytest.warns without asserting anything about the warnings, pass None as the expected warning type:

with pytest.warns(None) as record:
    warnings.warn("user", UserWarning)
    warnings.warn("runtime", RuntimeWarning)

assert len(record) == 2
assert str(record[0].message) == "user"
assert str(record[1].message) == "runtime"

The recwarn fixture will record warnings for the whole function:

import warnings

def test_hello(recwarn):
    warnings.warn("hello", UserWarning)
    assert len(recwarn) == 1
    w = recwarn.pop(UserWarning)
    assert issubclass(w.category, UserWarning)
    assert str(w.message) == "hello"
    assert w.filename
    assert w.lineno

Both recwarn and pytest.warns return the same interface for recorded warnings: a WarningsRecorder instance. To view the recorded warnings, you can iterate over this instance, call len on it to get the number of recorded warnings, or index into it to get a particular recorded warning.

Full API: WarningsRecorder.

Custom failure messages

Recording warnings provides an opportunity to produce custom test failure messages for when no warnings are issued or other conditions are met.

def test():
    with pytest.warns(Warning) as record:
        if not record:
            pytest.fail("Expected a warning!")

If no warnings are issued when calling f, then not record will evaluate to True. You can then call pytest.fail with a custom error message.

Internal pytest warnings

pytest may generate its own warnings in some situations, such as improper usage or deprecated features.

For example, pytest will emit a warning if it encounters a class that matches python_classes but also defines an __init__ constructor, as this prevents the class from being instantiated:

# content of test_pytest_warnings.py
class Test:
    def __init__(self):

    def test_foo(self):
        assert 1 == 1
$ pytest test_pytest_warnings.py -q

============================= warnings summary =============================
  $REGENDOC_TMPDIR/test_pytest_warnings.py:1: PytestCollectionWarning: cannot collect test class 'Test' because it has a __init__ constructor (from: test_pytest_warnings.py)
    class Test:

-- Docs: https://docs.pytest.org/en/latest/warnings.html
1 warning in 0.12s

These warnings might be filtered using the same builtin mechanisms used to filter other types of warnings.

Please read our Backwards Compatibility Policy to learn how we proceed about deprecating and eventually removing features.

The following warning types are used by pytest and are part of the public API:

class PytestWarning

Bases: UserWarning.

Base class for all warnings emitted by pytest.

class PytestAssertRewriteWarning

Bases: PytestWarning.

Warning emitted by the pytest assert rewrite module.

class PytestCacheWarning

Bases: PytestWarning.

Warning emitted by the cache plugin in various situations.

class PytestCollectionWarning

Bases: PytestWarning.

Warning emitted when pytest is not able to collect a file or symbol in a module.

class PytestConfigWarning

Bases: PytestWarning.

Warning emitted for configuration issues.

class PytestDeprecationWarning

Bases: pytest.PytestWarning, DeprecationWarning.

Warning class for features that will be removed in a future version.

class PytestExperimentalApiWarning

Bases: pytest.PytestWarning, FutureWarning.

Warning category used to denote experiments in pytest. Use sparingly as the API might change or even be removed completely in future version

class PytestUnhandledCoroutineWarning

Bases: PytestWarning.

Warning emitted when pytest encounters a test function which is a coroutine, but it was not handled by any async-aware plugin. Coroutine test functions are not natively supported.

class PytestUnknownMarkWarning

Bases: PytestWarning.

Warning emitted on use of unknown markers. See https://docs.pytest.org/en/latest/mark.html for details.