Use duck-typing to check for non-numeric types in approx()

Fix pytest-dev#8132

Author: nicoddemus

changelog/8132.bugfix.rst

@@ -0,0 +1,6 @@
+Fixed regression in ``approx``: in 6.2.0 ``approx`` no longer raises
+``TypeError`` when dealing with non-numeric types, falling back to normal comparison,
+however the check was done using ``isinstance`` which left out types which implemented
+the necessary methods for ``approx`` to work, such as tensorflow's ``DeviceArray``.
+
+The code has been changed to check for the necessary methods to accommodate those cases.

src/_pytest/python_api.py

@@ -241,12 +241,17 @@ def __eq__(self, actual) -> bool:
         if actual == self.expected:
             return True
 
-        # If either type is non-numeric, fall back to strict equality.
-        # NB: we need Complex, rather than just Number, to ensure that __abs__,
-        # __sub__, and __float__ are defined.
+        # Check types are non-numeric using duck-typing; if they are not numeric types,
+        # we consider them unequal because the short-circuit above failed.
+        required_attrs = [
+            "__abs__",
+            "__float__",
+            "__rsub__",
+            "__sub__",
+        ]
         if not (
-            isinstance(self.expected, (Complex, Decimal))
-            and isinstance(actual, (Complex, Decimal))
+            all(hasattr(self.expected, attr) for attr in required_attrs)
+            and all(hasattr(actual, attr) for attr in required_attrs)
         ):
             return False
 

testing/python/approx.py

@@ -6,6 +6,8 @@
 from operator import ne
 from typing import Optional
 
+import attr
+
 import pytest
 from _pytest.pytester import Pytester
 from pytest import approx
@@ -582,3 +584,37 @@ def __len__(self):
 
         expected = MySizedIterable()
         assert [1, 2, 3, 4] == approx(expected)
+
+    def test_duck_typing(self):
+        """
+        Check that approx() works for objects which implemented the required
+        numeric methods (#8132).
+        """
+
+        @attr.s(auto_attribs=True)
+        class Container:
+            value: float
+
+            def __abs__(self) -> float:
+                return abs(self.value)
+
+            def __sub__(self, other):
+                if isinstance(other, Container):
+                    return Container(self.value - other.value)
+                elif isinstance(other, (float, int)):
+                    return self.value - other
+                return NotImplemented
+
+            def __rsub__(self, other):
+                if isinstance(other, Container):
+                    return other.value - self.value
+                elif isinstance(other, (float, int)):
+                    return other - self.value
+                return NotImplemented
+
+            def __float__(self) -> float:
+                return self.value
+
+        assert Container(1.0) == approx(1 + 1e-7, rel=5e-7)
+        assert Container(1.0) != approx(1 + 1e-7, rel=1e-8)
+        assert Container(1.0) == approx(1.0)