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.
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).
Note
DeprecationWarning and PendingDeprecationWarning are treated differently; see Ensuring a function triggers a deprecation warning.
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. It also provides these methods:
A context manager to record raised warnings.
Adapted from warnings.catch_warnings.
Each recorded warning has the attributes message, category, filename, lineno, file, and line. The category is the class of the warning. The message is the warning itself; calling str(message) will return the actual message of the warning.
Note
DeprecationWarning and PendingDeprecationWarning are treated differently; see 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.simplefilter('always')
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():
myobject.deprecated_method()