Asserting Warnings¶
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.
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).
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. It also
provides these methods:
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.
Ensuring a function triggers a deprecation warning¶
You can also call a global helper for checking
that a certain function call triggers a DeprecationWarning
:
import pytest
def test_global():
pytest.deprecated_call(myfunction, 17)
By default, deprecation warnings will not be caught when using pytest.warns
or recwarn
, since the default Python warnings filters hide
DeprecationWarnings. 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)