From c387c1c3116d7d1de9698b8fff0fc64a90a33205 Mon Sep 17 00:00:00 2001
From: L-zeus <1956795691@qq.com>
Date: Wed, 28 Oct 2026 12:01:06 +0800
Subject: [PATCH 1/3] Fix overflow in NumPy integer approx failure details

Unbox the actual array scalar before mismatch and difference calculations, matching the existing equality path.

Implementation and local verification performed with OpenAI Codex assistance; pending human review.
---
 src/_pytest/approx.py    |  5 +++-
 testing/python/approx.py | 49 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/src/_pytest/approx.py b/src/_pytest/approx.py
index 10c88d42334..4520b2ef4af 100644
--- a/src/_pytest/approx.py
+++ b/src/_pytest/approx.py
@@ -220,7 +220,10 @@ def get_value_from_nested_list(
         different_ids = []
         for index in itertools.product(*(range(i) for i in np_array_shape)):
             approx_value = get_value_from_nested_list(approx_side_as_seq, index)
-            other_value = get_value_from_nested_list(other_side_as_array, index)
+            # As in _yield_comparisons, avoid overflow in numpy integer arithmetic.
+            other_value = _unbox_numpy_scalar(
+                get_value_from_nested_list(other_side_as_array, index)
+            )
             if approx_value != other_value:
                 abs_diff = abs(approx_value.expected - other_value)
                 max_abs_diff = _max_diff(max_abs_diff, abs_diff)
diff --git a/testing/python/approx.py b/testing/python/approx.py
index 35adcfea140..a9b6720149c 100644
--- a/testing/python/approx.py
+++ b/testing/python/approx.py
@@ -259,6 +259,55 @@ def test_error_messages_numpy_dtypes(self, assert_approx_raises_regex):
             ],
         )
 
+    @pytest.mark.parametrize(
+        ("dtype", "actual", "expected", "absolute_difference", "relative_difference"),
+        [
+            ("uint8", 255, 0, 255, 1.0),
+            ("int8", -128, 127, 255, 1.9921875),
+            ("uint64", 2**64 - 1, 0, 2**64 - 1, 1.0),
+            ("int64", -(2**63), 2**63 - 1, 2**64 - 1, 2.0),
+        ],
+    )
+    def test_numpy_integer_failure_details(
+        self, dtype, actual, expected, absolute_difference, relative_difference
+    ) -> None:
+        np = pytest.importorskip("numpy")
+        actual_array = np.array([actual], dtype=dtype)
+        expected_array = np.array([expected], dtype=dtype)
+        comparison = approx(expected_array)
+
+        assert actual_array != comparison
+        explanation = comparison._repr_compare(actual_array)
+        assert explanation[:3] == [
+            "comparison failed. Mismatched elements: 1 / 1:",
+            f"Max absolute difference: {absolute_difference}",
+            f"Max relative difference: {relative_difference}",
+        ]
+
+    def test_numpy_integer_assertion_details(self, pytester: Pytester) -> None:
+        pytest.importorskip("numpy")
+        pytester.makepyfile(
+            """
+            import numpy as np
+            import pytest
+
+            def test_difference():
+                assert np.array([255], dtype=np.uint8) == pytest.approx(
+                    np.array([0], dtype=np.uint8)
+                )
+            """
+        )
+        result = pytester.runpytest("-Werror")
+        result.assert_outcomes(failed=1)
+        result.stdout.fnmatch_lines(
+            [
+                "*Mismatched elements: 1 / 1:*",
+                "*Max absolute difference: 255*",
+                "*Max relative difference: 1.0*",
+            ]
+        )
+        assert "representation of details failed" not in result.stdout.str()
+
     def test_error_messages_invalid_args(self, assert_approx_raises_regex):
         np = pytest.importorskip("numpy")
         with pytest.raises(AssertionError) as e:

From 8992fdf333989f5efa4653b1d0a0a358e1b41f23 Mon Sep 17 00:00:00 2001
From: L-zeus <1956795691@qq.com>
Date: Sun, 1 Nov 2026 10:13:56 +0800
Subject: [PATCH 2/3] docs: credit contributor for NumPy integer fix

Codex assisted the implementation and verification of the NumPy integer
failure-diagnostics fix. L-zeus reviewed the changes and reproduced the bug.

Co-authored-by: Codex <noreply@openai.com>
---
 AUTHORS | 1 +
 1 file changed, 1 insertion(+)

diff --git a/AUTHORS b/AUTHORS
index 136ffd6c5ee..46db9a4e322 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -296,6 +296,7 @@ lovetheguitar
 Luana Pinho Bueno Penha
 Lukas Bednar
 Luke Murphy
+L-zeus
 Maciek Fijalkowski
 Madan Kumar
 Maggie Chung

From 3c373ef441f649d9a811a72251f0c79882423ea6 Mon Sep 17 00:00:00 2001
From: L-zeus <1956795691@qq.com>
Date: Sun, 1 Nov 2026 10:38:39 +0800
Subject: [PATCH 3/3] docs: add changelog for NumPy integer approx diagnostics

---
 changelog/15312.bugfix.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog/15312.bugfix.rst

diff --git a/changelog/15312.bugfix.rst b/changelog/15312.bugfix.rst
new file mode 100644
index 00000000000..2b31c220a8c
--- /dev/null
+++ b/changelog/15312.bugfix.rst
@@ -0,0 +1 @@
+Fixed incorrect difference values and integer overflow warnings in :func:`pytest.approx` failure messages for NumPy integer arrays.