fix: approx() tolerance not applied to sequences/mappings nested inside dicts

[Ashutosh-177]

Running into an issue where pytest.approx works fine at the top level but silently falls back to exact equality when the same sequence or mapping appears as a dict value.

For example:

assert {"a": [1.0, 2.0 + 1e-7]} == pytest.approx({"a": [1.0, 2.0]}, abs=1e-5)

This fails even though the tolerance should cover the difference. The top-level approx() call dispatches to ApproxMapping, which iterates over the dict values and calls _approx_scalar() on each one. But _approx_scalar() only checks for Decimal and datetime/timedelta — everything else, including lists and nested dicts, falls through to ApproxScalar, which does exact comparison.

The top-level approx() dispatcher already has the correct isinstance checks for Mapping and sequence-like types. Adding the same two checks to _approx_scalar() makes nested structures recurse properly.

Fixes #8703

[Copilot]

Pull request overview

Note

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Extends _approx_scalar in python_api.py to dispatch Mapping and sequence-like values to ApproxMapping and ApproxSequenceLike respectively, enabling nested approximate comparisons within scalar dispatch.

Changes:

• Add Mapping branch to return ApproxMapping.
• Add sequence-like branch to return ApproxSequenceLike.

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[Pierre-Sassoulas]

Hello, thank you for working on pytest @Ashutosh-177, it seems your second commit canceled the first, so there's nothing to review.

[bluetech]

Thanks, but as said in the issue, nested structures are currently not supported, in fact it's a bug that the case you give doesn't throw an error. If we want to support them, a more thorough fix will be needed.

If I change your example to fail (e.g. change abs=1e-8), the repr fails:

E   AssertionError: assert {'a': [1.0, 2.0000001]} == approx({'a': ... ± 1.0e-08])})
E     
E     (pytest_assertion plugin: representation of details failed: /home/ran/src/pytest/src/_pytest/python_api.py:274: TypeError: unsupported operand type(s) for -: 'list' and 'list'.
E      Probably an object has a faulty __repr__.)