Unbox numpy scalars in sequence comparison

AUTHORS

@@ -426,6 +426,7 @@ Raquel Alegre
 Ravi Chandra
 Reagan Lee
 Reilly Brogan
+rinopatrick
 Rob Arrow
 Robert Holt
 Roberto Aldera

changelog/15314.bugfix.rst

@@ -0,0 +1 @@
+Fixed :func:`pytest.approx` reporting wrong differences (and wrong equality results) when comparing a list expected value against a NumPy integer array: NumPy scalars are now unboxed before subtraction, avoiding wraparound such as ``0 - np.uint8(1)`` evaluating to ``255``.

src/_pytest/approx.py

@@ -431,6 +431,9 @@ def _repr_compare(self, other_side: Sequence[float]) -> list[str]:
         for i, (approx_value, other_value) in enumerate(
             zip(approx_side_as_map, other_side, strict=True)
         ):
+            # Unbox numpy scalars first: e.g. `0 - np.uint8(1)` wraps around
+            # to 255 instead of -1 (see #15314).
+            other_value = _unbox_numpy_scalar(other_value)
             if approx_value != other_value:
                 try:
                     abs_diff = abs(approx_value.expected - other_value)
@@ -477,7 +480,7 @@ def __eq__(self, actual) -> bool:
         return super().__eq__(actual)
 
     def _yield_comparisons(self, actual):
-        return zip(actual, self.expected, strict=True)
+        return zip((_unbox_numpy_scalar(a) for a in actual), self.expected, strict=True)
 
 
 class ApproxScalar(Approx[ExpectedT]):

testing/python/approx.py

@@ -259,6 +259,16 @@ def test_error_messages_numpy_dtypes(self, assert_approx_raises_regex):
             ],
         )
 
+    def test_numpy_integer_actual_against_list_expected(self):
+        # Numpy integer scalars must be unboxed before comparison, otherwise
+        # e.g. `0 - np.uint8(1)` wraps around to 255 (see #15314).
+        np = pytest.importorskip("numpy")
+
+        assert np.array([1], dtype=np.uint8) == pytest.approx([0], abs=5)
+        assert np.array([0], dtype=np.uint8) == pytest.approx([1], abs=5)
+        assert np.array([255], dtype=np.uint8) != pytest.approx([0])
+        assert np.array([-128], dtype=np.int8) != pytest.approx([127])
+
     def test_error_messages_invalid_args(self, assert_approx_raises_regex):
         np = pytest.importorskip("numpy")
         with pytest.raises(AssertionError) as e: