Improve parametrize error for scalar parameter sets#14621
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Tuple-style parametrization expects each parameter set to be a sequence of values. When a scalar like None was passed, pytest crashed while trying to format the validation error because it called len() on the scalar. Report a normal collection error instead, pointing at the offending test. Tested: PYTHONPATH=src python3 -m pytest testing/test_mark.py::test_parametrized_collect_with_non_sequence_values testing/test_mark.py::test_parametrized_collect_with_wrong_args -q Tested: PYTHONPATH=src python3 -m pytest testing/test_mark.py -q Tested: git diff --check -- src/_pytest/mark/structures.py testing/test_mark.py changelog/14619.bugfix.rst AUTHORS Tested: python3 -m py_compile src/_pytest/mark/structures.py testing/test_mark.py Not-tested: testing/python/metafunc.py because hypothesis is not installed in this environment. Co-authored-by: OpenAI Codex <codex@openai.com>
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DTiming24
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Jun 25, 2026
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Good improvement turning len(None) into a collection error. Since the repro in the body uses @pytest.mark.parametrize("x,", [None]), I think the error message should probably mention the single-value tuple syntax explicitly (('value',)), because that's the quickest fix users can make.
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Closes #14619.
This turns a
TypeErrorduring collection into a normalparametrizecollection error when tuple-style argnames receive a scalar parameter set.For example,
@pytest.mark.parametrize("x,", [None])now points at the offending test and reports that the parameter set should be a sequence, instead of showing an internallen(None)crash.I added a regression test for the
Nonecase and a changelog entry.I used AI assistance while preparing this change, reviewed the patch locally, and take responsibility for the contribution.
Checks run locally:
testing/python/metafunc.pywas not run locally becausehypothesisis not installed in this environment.Checklist:
parametrizecauses obfuscated crash #14619.changelog/14619.bugfix.rst.AUTHORS.