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Asserting Warnings
=====================================================
.. _warns:
Asserting warnings with the warns function
-----------------------------------------------
.. versionadded:: 2.8
You can check that code raises a particular warning using ``pytest.warns``,
which works in a similar manner to :ref:`raises <assertraises>`::
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
:ref:`recwarn <recwarn>` fixture (see below).
.. note::
``DeprecationWarning`` and ``PendingDeprecationWarning`` are treated
differently; see :ref:`ensuring_function_triggers`.
.. _recwarn:
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:
.. autoclass:: _pytest.recwarn.WarningsRecorder()
:members:
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 :ref:`ensuring_function_triggers`.
.. _ensuring_function_triggers:
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()
|
