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 +1,5 @@
+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

@@ -894,7 +894,9 @@ def execute(self, request):
         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
+            if my_cache_key is cache_key:
                 if err is not None:
                     _, val, tb = err
                     raise val.with_traceback(tb)