fix: improve error message when pytest.approx() dicts have different keys

[enioxt]

Description

Fixes #13816

This PR improves the error message when using pytest.approx() to compare dictionaries with different keys.

Problem

When dictionaries have different keys, the current error message is confusing:

AssertionError: assert approx({'a': ... 4 ± 4.0e-06}) == {'a': 1, 'c': 3}
(pytest_assertion plugin: representation of details failed: KeyError: 'b'.
Probably an object has a faulty __repr__.)

This suggests there's a problem with __repr__, but the real issue is just that the dict keys don't match.

Solution

Added a clear check for key mismatch before attempting value comparison:

expected_keys = set(self.expected.keys())
other_keys = set(other_side.keys())
if expected_keys \!= other_keys:
    # Return clear error message

New Error Message

AssertionError: Dictionaries have different keys.
Expected keys: ['a', 'c'], Actual keys: ['a', 'b']

Changes

File: src/_pytest/python_api.py

• Added key comparison check in ApproxMapping._repr_compare()
• Returns early with clear message if keys don't match
• Shows which keys are in expected vs actual

File: testing/test_approx_dict_keys_13816.py (NEW)

• 4 focused test cases covering different scenarios

✅ Testing

def test_approx_dicts_with_different_keys_clear_error():
    expected = {"a": 1, "c": 3}
    actual = {"a": 1, "b": 4}
    
    with pytest.raises(AssertionError) as exc_info:
        assert pytest.approx(actual) == expected
    
    error_message = str(exc_info.value)
    assert "different keys" in error_message.lower()  # ✅
    assert "KeyError" not in error_message  # ✅
    assert "faulty __repr__" not in error_message  # ✅

Test Cases:

• Different keys → Clear error message
• Extra key in expected → Detected
• Extra key in actual → Detected
• Matching keys → Normal approx behavior still works

Benefits

• Clearer UX: Users immediately understand the issue
• No false leads: Eliminates confusing KeyError/__repr__ messages
• Minimal change: Only affects error path, no behavior changes
• Well-tested: 4 test cases cover edge cases

Sacred Code: 000.111.369.963.1618

[nicoddemus]

Thanks, superseded by #13818.