Pytest-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():

Running pytest now produces this output:

$ pytest test_show_warnings.py
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 1 item

test_show_warnings.py .                                              [100%]

test_show_warnings.py::test_one
  $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/stable/warnings.html

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%]
_________________________________ test_one _________________________________

    def test_one():

test_show_warnings.py:10:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    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
FAILED test_show_warnings.py::test_one - UserWarning: api v1, should use ...
1 failed in 0.12s

The same option can be set in the pytest.ini or pyproject.toml file
using the filterwarnings ini option. For example, the configuration
below will ignore all user warnings and specific deprecation warnings
matching a regex, but will transform all other warnings into errors.

# pytest.ini
[pytest]
filterwarnings =
    error
    ignore::UserWarning
    ignore:function ham\(\) is deprecated:DeprecationWarning
# pyproject.toml
[tool.pytest.ini_options]
filterwarnings = [
    "error",
    "ignore::UserWarning",
    # note the use of single quote below to denote "raw" strings in TOML
    'ignore:function ham\(\) is deprecated:DeprecationWarning',
]

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.

@pytest.mark.filterwarnings {#filterwarnings}

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():

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{.interpreted-text role=“globalvar”}
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:

[pytest]
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 {#deprecation-warnings}

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:

[pytest]
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".

::: {.note}
::: {.title}
Note
:::

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{#ensuring a function triggers a deprecation warning} {#ensuring_function_triggers}

You can also use pytest.deprecated_call{.interpreted-text role=“func”}
for checking that a certain function call triggers a
DeprecationWarning or PendingDeprecationWarning:

import pytest


def test_myfunction_deprecated():
    with pytest.deprecated_call():
        myfunction(17)

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{.interpreted-text role=“func”}
or recwarn <recwarn>{.interpreted-text role=“ref”} 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):
    warnings.simplefilter("always")
    myfunction(17)
    assert recwarn.pop(DeprecationWarning)

The recwarn <recwarn>{.interpreted-text role=“ref”} 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{#asserting warnings with the warns function}{#assertwarnings}{#asserting warnings} {#warns}

You can check that code raises a particular warning using
func:[pytest.warns]{.title-ref}, which works in a similar manner to
raises <assertraises>{.interpreted-text role=“ref”}:

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 func:[pytest.warns]{.title-ref} 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
# check that the message matches

Alternatively, you can examine raised warnings in detail using the
recwarn <recwarn>{.interpreted-text role=“ref”} fixture (see below).

::: {.note}
::: {.title}
Note
:::

DeprecationWarning and PendingDeprecationWarning are treated
differently; see ensuring_function_triggers{.interpreted-text
role=“ref”}.
:::

Recording warnings{#recording warnings} {#recwarn}

You can record raised warnings either using
func:[pytest.warns]{.title-ref} or with the recwarn fixture.

To record with func:[pytest.warns]{.title-ref} 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)

The recwarn fixture will record warnings for the whole function:

import warnings


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

Both recwarn and func:[pytest.warns]{.title-ref} 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.

::: {.currentmodule}
_pytest.warnings
:::

Full API: ~_pytest.recwarn.WarningsRecorder{.interpreted-text
role=“class”}.

Custom failure messages {#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:
        f()
        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{.interpreted-text
role=“func”} with a custom error message.

Internal pytest warnings {#internal-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{.interpreted-text role=“confval”} 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):
        pass

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

test_pytest_warnings.py:1
  $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/stable/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{.interpreted-text role=“ref”}
to learn how we proceed about deprecating and eventually removing
features.

The full list of warnings is listed in
the reference documentation <warnings ref>{.interpreted-text
role=“ref”}.

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