Testing warnings#

Testfixtures has tools that make it easy to make assertions about code that may emit warnings.

The ShouldWarn context manager#

This context manager allows you to assert that particular warnings are emitted in a block of code, for example:

>>> from warnings import warn
>>> from testfixtures import ShouldWarn
>>> with ShouldWarn(UserWarning('you should fix that')):
...     warn('you should fix that')

If a warning issued doesn’t match the one expected, ShouldWarn will raise an AssertionError causing the test in which it occurs to fail:

>>> from warnings import warn
>>> from testfixtures import ShouldWarn
>>> with ShouldWarn(UserWarning('you should fix that')):
...     warn("sorry dave, I can't let you do that")
Traceback (most recent call last):
...
AssertionError: sequence not as expected:

same:
[]

expected:
[
<C:....UserWarning(failed)>
attributes differ:
'args': ('you should fix that',) (Comparison) != ("sorry dave, I can't let you do that",) (actual)
</C:....UserWarning>]

actual:
[UserWarning("sorry dave, I can't let you do that"...)]

You can check multiple warnings in a particular piece of code:

>>> from warnings import warn
>>> from testfixtures import ShouldWarn
>>> with ShouldWarn(UserWarning('you should fix that'),
...                 UserWarning('and that too')):
...     warn('you should fix that')
...     warn('and that too')

If you want to inspect more details of the warnings issued, you can capture them into a list as follows:

>>> from warnings import warn_explicit
>>> from testfixtures import ShouldWarn
>>> with ShouldWarn() as captured:
...     warn_explicit(message='foo', category=DeprecationWarning,
...                   filename='bar.py', lineno=42)
>>> len(captured)
1
>>> captured[0].message
DeprecationWarning('foo'...)
>>> captured[0].lineno
42

The ShouldNotWarn context manager#

If you do not expect any warnings to be logged in a piece of code, you can use the ShouldNotWarn context manager. If any warnings are issued in the context it manages, it will raise an AssertionError to indicate this:

>>> from warnings import warn
>>> from testfixtures import ShouldNotWarn
>>> with ShouldNotWarn():
...     warn("woah dude")
Traceback (most recent call last):
...
AssertionError: sequence not as expected:

same:
[]

expected:
[]

actual:
[UserWarning('woah dude'...)]