fix pytest-dev#11797: be more lenient on SequenceLike approx

this needs a validation as it allows partially implemented sequences

changelog/11797.bugfix.rst

@@ -0,0 +1 @@
+Ensure approx for SequenceLike objects doesn't wrap those sequences in a scalar.

src/_pytest/python_api.py

@@ -129,6 +129,8 @@ def _recursive_sequence_map(f, x):
     if isinstance(x, (list, tuple)):
         seq_type = type(x)
         return seq_type(_recursive_sequence_map(f, xi) for xi in x)
+    elif _is_sequence_like(x):
+        return [_recursive_sequence_map(f, xi) for xi in x]
     else:
         return f(x)
 
@@ -721,11 +723,7 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
     elif _is_numpy_array(expected):
         expected = _as_numpy_array(expected)
         cls = ApproxNumpy
-    elif (
-        hasattr(expected, "__getitem__")
-        and isinstance(expected, Sized)
-        and not isinstance(expected, (str, bytes))
-    ):
+    elif _is_sequence_like(expected):
         cls = ApproxSequenceLike
     elif isinstance(expected, Collection) and not isinstance(expected, (str, bytes)):
         msg = f"pytest.approx() only supports ordered sequences, but got: {expected!r}"
@@ -736,6 +734,15 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
     return cls(expected, rel, abs, nan_ok)
 
 
+def _is_sequence_like(expected: object) -> bool:
+    return (
+        hasattr(expected, "__getitem__")
+        and isinstance(expected, Sized)
+        # Type ignored because the error is wrong -- not unreachable.
+        and not isinstance(expected, STRING_TYPES)  # type: ignore[unreachable]
+    )
+
+
 def _is_numpy_array(obj: object) -> bool:
     """
     Return true if the given object is implicitly convertible to ndarray,

testing/python/approx.py

@@ -954,6 +954,44 @@ def test_allow_ordered_sequences_only(self) -> None:
         with pytest.raises(TypeError, match="only supports ordered sequences"):
             assert {1, 2, 3} == approx({1, 2, 3})
 
+    def test_strange_sequence(self):
+        """https://github.com/pytest-dev/pytest/issues/11797"""
+
+        a = MyVec3(1, 2, 3)
+        b = MyVec3(0, 1, 2)
+
+        # this would trigger the error inside the test
+        pytest.approx(a, abs=0.5)._repr_compare(b)
+
+        assert b == pytest.approx(a, abs=2)
+        assert b != pytest.approx(a, abs=0.5)
+
+
+class MyVec3: #incomplete
+    """sequence like"""
+
+    _x: int
+    _y: int
+    _z: int
+
+    def __init__(self, x: int, y: int, z: int):
+        self._x, self._y, self._z = x, y, z
+
+    def __repr__(self) -> str:
+        return f"<MyVec3 {self._x} {self._y} {self._z}>"
+
+    def __len__(self) -> int:
+        return 3
+
+    def __getitem__(self, key: int) -> int:
+        if key == 0:
+            return self._x
+        if key == 1:
+            return self._y
+        if key == 2:
+            return self._z
+        raise IndexError(key)
+
 
 class TestRecursiveSequenceMap:
     def test_map_over_scalar(self):
@@ -981,3 +1019,6 @@ def test_map_over_mixed_sequence(self):
             (5, 8),
             [(7)],
         ]
+
+    def test_map_over_sequence_like(self):
+        assert _recursive_sequence_map(int, MyVec3(1, 2, 3)) == [1, 2, 3]