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pytest.approx loses mismatch details for same-key mappings with unequal strings #15009
Description
pytest.approx supports non-numeric mapping values, but unequal strings under the same key cause its assertion diagnostic formatter to raise TypeError.
Save as test_approx_mapping.py:
import pytest
def test_mapping_string_mismatch():
assert {"item": "a"} == pytest.approx({"item": "b"})Run python -m pytest -q test_approx_mapping.py.
The assertion fails as expected, but its details are replaced by this message (local source path omitted):
(pytest_assertion plugin: representation of details failed: ... TypeError: unsupported operand type(s) for -: 'str' and 'str'.
Probably an object has a faulty __repr__.)
Expected: a failed assertion with mapping mismatch details and no internal formatter exception. The equivalent unequal-string sequence retains its detail table.
#13010 and #13012 cover sequences; #13816 concerns different mapping keys. This example uses identical keys.
Reproduced on pytest 9.1.1 and development commit 3fd8675d6d798507c06cf9c60753be6d9d7b0e17, with CPython 3.12.3 on Linux 6.8.0-100-generic x86-64.
Installed packages (uv pip list --offline; this environment has no pip module):
Package Version
---------------- -------
attrs 26.1.0
hypothesis 6.168.0
iniconfig 2.3.0
numpy 2.5.3
packaging 26.3
pluggy 1.6.0
pygments 2.21.0
pytest 9.1.1
sortedcontainers 2.4.0
GPT-6 Astra assisted; I manually reviewed this report.
Activity
deepak7lal commentedon Sep 13, 2026
I'd like to take this. Reproduced on
main, and the cause is a small asymmetry between the mapping and sequence formatters.ApproxMapping._repr_compare(src/_pytest/approx.py) guards the diff arithmetic with:while
ApproxSequenceLike._repr_compareguards the equivalent block withexcept TypeError, added in #13012 for #13010. Subtracting two strings raisesTypeError, so the mapping path lets it escape into the assertion-repr hook, which is why the details are replaced by the "representation of details failed" message while the sequence equivalent still prints its table:Plan, following #13012:
except (ZeroDivisionError, TypeError)so non-numeric values are skipped for the max-abs/max-rel calculations but still reported in the mismatch table.testing/python/approx.pyalongsidetest_mixed_sequence, covering both an all-string mapping and a mixed numeric/string mapping.changelog/entry.I'll keep #15010 out of scope since that one is about the Decimal tolerance repr rather than the formatter raising.
Fix pytest.approx mapping details for non-numeric values