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Pytest 8.4 pytest.PytestReturnNotNoneWarning is now an exception #241
Description
The recent release of pytest 8.4 has begun to cause failures in the pytest_mpl tests in the networkx test suite (networkx/networkx#8090).
AIUI, the crux of the problem is that pytest now raises whenever a test returns a non-None value, which conflicts with the design of the mpl_image_compare mark. See also pytest-dev/pytest#13477 for further context/links/discussion.
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ConorMacBride commentedon Jun 3, 2025
Thanks @rossbar for letting us know that this warning is now an exception. There are some more details in this section of the docs, but basically when pytest-mpl is installed it'll always intercept any returned figures and prevent this exception from being raised. Alternatively the tests could be skipped. (See also #183)
ConorMacBride commentedon Jun 30, 2025
Closing as this behavior has now been reverted in Pytest 8.4.1 pytest-dev/pytest#13495