Warnings Capture

New in version 3.1.

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-3.x.y, py-1.x.y, pluggy-0.x.y
rootdir: $REGENDOC_TMPDIR, inifile:
collected 1 item

test_show_warnings.py .                                              [100%]

============================= warnings summary =============================
  $REGENDOC_TMPDIR/test_show_warnings.py:4: UserWarning: api v1, should use functions from v2
    warnings.warn(UserWarning("api v1, should use functions from v2"))

-- Docs: http://doc.pytest.org/en/latest/warnings.html
=================== 1 passed, 1 warnings in 0.12 seconds ===================

Pytest by default catches all warnings except for DeprecationWarning and PendingDeprecationWarning.

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:4: UserWarning
1 failed in 0.12 seconds

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.


New in version 3.2.

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")


Except for these features, pytest does not change the python warning filter; it only captures and displays the warnings which are issued with respect to the currently configured filter, including changes to the filter made by test functions or by the system under test.


DeprecationWarning and PendingDeprecationWarning are hidden by the standard library by default so you have to explicitly configure them to be displayed in your pytest.ini:

filterwarnings =

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

Disabling warning capture

This feature 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.

Asserting warnings with the warns function

New in version 2.8.

You can check that code raises a particular warning using pytest.warns, which works in a similar manner 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 to assert that the exception matches a text or regex:

>>> 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):
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 a function 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.

Ensuring a function triggers a deprecation warning

You can also call a global helper for checking that a certain function call triggers a DeprecationWarning or PendingDeprecationWarning:

import pytest

def test_global():
    pytest.deprecated_call(myfunction, 17)

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

import warnings
import pytest

def test_deprecation(recwarn):
    warnings.warn("deprecated", DeprecationWarning)
    assert len(recwarn) == 1
    assert recwarn.pop(DeprecationWarning)

You can also use it as a contextmanager:

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