Replace `==` with `is` for comparison of cache keys

Closes #6497

AUTHORS

@@ -112,6 +112,7 @@ Guido Wesdorp
 Guoqiang Zhang
 Harald Armin Massa
 Henk-Jaap Wagenaar
+Holger Kohr
 Hugo van Kemenade
 Hui Wang (coldnight)
 Ian Bicking

changelog/6497.bugfix.rst

@@ -0,0 +1,4 @@
+Fix bug in the comparison of request key with cached key in fixture.
+
+A construct ``if key == cached_key:`` can fail either because ``==`` is explicitly disallowed, or for, e.g., NumPy arrays, where the result of ``a == b`` cannot generally be converted to `bool`.
+The implemented fix replaces `==` with ``is``.

src/_pytest/fixtures.py

@@ -898,7 +898,9 @@ class FixtureDef:
         cached_result = getattr(self, "cached_result", None)
         if cached_result is not None:
             result, cache_key, err = cached_result
-            if my_cache_key == cache_key:
+            # note: comparison with `==` can fail (or be expensive) for e.g.
+            # numpy arrays (#6497)
+            if my_cache_key is cache_key:
                 if err is not None:
                     _, val, tb = err
                     raise val.with_traceback(tb)

testing/python/fixtures.py

@@ -1102,6 +1102,38 @@ class TestFixtureUsages:
             "*Fixture 'badscope' from test_invalid_scope.py got an unexpected scope value 'functions'"
         )
 
+    @pytest.mark.parametrize("scope", ["function", "session"])
+    def test_parameters_without_eq_semantics(self, scope, testdir):
+        testdir.makepyfile(
+            """
+            class NoEq1:  # fails on `a == b` statement
+                def __eq__(self, _):
+                    raise RuntimeError
+
+            class NoEq2:  # fails on `if a == b:` statement
+                def __eq__(self, _):
+                    class NoBool:
+                        def __bool__(self):
+                            raise RuntimeError
+                    return NoBool()
+
+            import pytest
+            @pytest.fixture(params=[NoEq1(), NoEq2()], scope={scope!r})
+            def no_eq(request):
+                return request.param
+
+            def test1(no_eq):
+                pass
+
+            def test2(no_eq):
+                pass
+        """.format(
+                scope=scope
+            )
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*4 passed*"])
+
     def test_funcarg_parametrized_and_used_twice(self, testdir):
         testdir.makepyfile(
             """