pytest.approx loses mismatch details for same-key mappings with unequal strings

[torjan0]

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.

[deepak7lal]

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:

except ZeroDivisionError:
    pass

while ApproxSequenceLike._repr_compare guards the equivalent block with except TypeError, added in #13012 for #13010. Subtracting two strings raises TypeError, 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:

# mapping  -> TypeError: unsupported operand type(s) for -: 'str' and 'str'
# sequence -> Index | Obtained | Expected
#             1     | a        | b

Plan, following #13012:

• Widen the guard to except (ZeroDivisionError, TypeError) so non-numeric values are skipped for the max-abs/max-rel calculations but still reported in the mismatch table.
• Add a mapping test in testing/python/approx.py alongside test_mixed_sequence, covering both an all-string mapping and a mixed numeric/string mapping.
• Add a changelog/ entry.

I'll keep #15010 out of scope since that one is about the Decimal tolerance repr rather than the formatter raising.