How to capture warnings¶
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-7.x.y, pluggy-1.x.y rootdir: /home/sweet/project collected 1 item test_show_warnings.py . [100%] ============================= warnings summary ============================= test_show_warnings.py::test_one /home/sweet/project/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/how-to/capture-warnings.html ======================= 1 passed, 1 warning in 0.12s =======================
Similar to Python’s warning filter and
-W option flag, pytest provides
-W flag to control which warnings are ignored, displayed, or turned into
errors. See the warning filter documentation for more
This code sample shows how to treat any
UserWarning category class of warning
as an error:
$ pytest -q test_show_warnings.py -W error::UserWarning F [100%] ================================= FAILURES ================================= _________________________________ test_one _________________________________ def test_one(): > assert api_v1() == 1 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 ========================= short test summary info ========================== 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
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.
-W flag and the
filterwarnings ini option use warning filters that are
similar in structure, but each configuration option interprets its filter
differently. For example, message in
filterwarnings is a string containing a
regular expression that the start of the warning message must match,
case-insensitively, while message in
-W is a literal string that the start of
the warning message must contain (case-insensitively), ignoring any whitespace at
the start or end of message. Consult the warning filter documentation for more
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
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
# 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
-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
PendingDeprecationWarning warnings from
user code and third-party libraries, as recommended by PEP 565.
This helps users keep their code modern and avoid breakages when deprecated warnings are effectively removed.
However, in the specific case where users capture any type of warnings in their test, either with
pytest.deprecated_call() or using the recwarn fixture,
no warning will be displayed at all.
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.
[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".
See @pytest.mark.filterwarnings and Controlling warnings for more examples.
If warnings are configured at the interpreter level, using
PYTHONWARNINGS environment variable or the
-W command-line option, pytest will not configure any filters by default.
Also pytest doesn’t follow PEP 506 suggestion of resetting all warning filters because
it might break test suites that configure warning filters themselves
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
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
Asserting warnings with the warns function¶
You can check that code raises a particular warning using
which works in a similar manner to raises (except that
raises does not capture all exceptions, only the
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. Use the keyword
match to assert that the warning matches a text or regex.
To match a literal string that may contain regular expression metacharacters like
., the pattern can
first be escaped with
>>> 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... were emitted... >>> with warns(UserWarning, match=re.escape("issue with foo() func")): ... warnings.warn("issue with foo() func") ...
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
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.message.args == "another warning"
Alternatively, you can examine raised warnings in detail using the recwarn fixture (see below).
The recwarn fixture automatically ensures to reset the warnings filter at the end of the test, so no global state is leaked.
You can record raised warnings either using
pytest.warns() or with
To record with
pytest.warns() without asserting anything about the warnings,
pass no arguments as the expected warning type and it will default to a generic Warning:
with pytest.warns() as record: warnings.warn("user", UserWarning) warnings.warn("runtime", RuntimeWarning) assert len(record) == 2 assert str(record.message) == "user" assert str(record.message) == "runtime"
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
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.
Additional use cases of warnings in tests¶
Here are some use cases involving warnings that often come up in tests, and suggestions on how to deal with them:
To ensure that at least one of the indicated warnings is issued, use:
def test_warning(): with pytest.warns((RuntimeWarning, UserWarning)): ...
To ensure that only certain warnings are issued, use:
def test_warning(recwarn): ... assert len(recwarn) == 1 user_warning = recwarn.pop(UserWarning) assert issubclass(user_warning.category, UserWarning)
To ensure that no warnings are emitted, use:
def test_warning(): with warnings.catch_warnings(): warnings.simplefilter("error") ...
To suppress warnings, use:
with warnings.catch_warnings(): warnings.simplefilter("ignore") ...
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
not record will
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
__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): assert 1 == 1
$ pytest test_pytest_warnings.py -q ============================= warnings summary ============================= test_pytest_warnings.py:1 /home/sweet/project/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/how-to/capture-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 full list of warnings is listed in the reference documentation.
Additional information of the source of a
ResourceWarning can be obtained when captured by pytest if
tracemalloc module is enabled.
One convenient way to enable
tracemalloc when running tests is to set the
PYTHONTRACEMALLOC to a large
enough number of frames (say
20, but that number is application dependent).
For more information, consult the Python Development Mode section in the Python documentation.