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python_api: improve approx assert message on mappings with only extra elements #13723

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Merged
merged 1 commit into from
Sep 16, 2025
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python_api: improve approx assert message on mappings with only extra elements #13723

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1 change: 1 addition & 0 deletions changelog/13722.bugfix.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fixed a misleading assertion failure message when using :func:`pytest.approx` on mappings with differing lengths.
6 changes: 6 additions & 0 deletions src/_pytest/python_api.py
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Expand Up @@ -236,6 +236,12 @@ def __repr__(self) -> str:
def _repr_compare(self, other_side: Mapping[object, float]) -> list[str]:
import math

if len(self.expected) != len(other_side):
return [
"Impossible to compare mappings with different sizes.",
f"Lengths: {len(self.expected)} and {len(other_side)}",
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If the bigger length is small (<10) and (or? Calling str might be costly) the Str representation of both is small (fit in max truncation) we might want to show the two mappings directly ?

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The full message does include a snippet:

    def test_it():
>       assert pytest.approx({"a": 1, "b": 2}) == {"a": 1, "b": 2, "c": 3}
E       AssertionError: assert approx({'a': ... 2 ± 2.0e-06}) == {'a': 1, 'b': 2, 'c': 3}
E         
E         Impossible to compare mappings with different sizes.
E         Lengths: 2 and 3

x.py:4: AssertionError

]

approx_side_as_map = {
k: self._approx_scalar(v) for k, v in self.expected.items()
}
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11 changes: 11 additions & 0 deletions testing/python/approx.py
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Expand Up @@ -741,6 +741,17 @@ def test_dict_for_div_by_zero(self, assert_approx_raises_regex):
],
)

def test_dict_differing_lengths(self, assert_approx_raises_regex):
assert_approx_raises_regex(
{"a": 0},
{"a": 0, "b": 1},
[
" ",
r" Impossible to compare mappings with different sizes\.",
r" Lengths: 2 and 1",
],
)

def test_numpy_array(self):
np = pytest.importorskip("numpy")

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