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src/_pytest/fixtures.py

@@ -241,14 +241,24 @@ def getfixturemarker(obj: object) -> Optional["FixtureFunctionMarker"]:
 
 
 @dataclasses.dataclass(frozen=True)
-class FixtureArgKey:
+class FixtureArgKeyByIndex:
     argname: str
-    param_index: Optional[int]
-    param_value: Optional[Hashable]
+    param_index: int
     scoped_item_path: Optional[Path]
     item_cls: Optional[type]
 
 
+@dataclasses.dataclass(frozen=True)
+class FixtureArgKeyByValue:
+    argname: str
+    param_value: Hashable
+    scoped_item_path: Optional[Path]
+    item_cls: Optional[type]
+
+
+FixtureArgKey = Union[FixtureArgKeyByIndex, FixtureArgKeyByValue]
+
+
 def get_parametrized_fixture_keys(
     item: nodes.Item, scope: Scope
 ) -> Iterator[FixtureArgKey]:
@@ -268,13 +278,6 @@ def get_parametrized_fixture_keys(
             if cs._arg2scope[argname] != scope:
                 continue
 
-            param_index: Optional[int] = cs.indices[argname]
-            param_value = cs.params[argname]
-            if isinstance(cs.params[argname], Hashable):
-                param_index = None
-            else:
-                param_value = None
-
             item_cls = None
             if scope is Scope.Session:
                 scoped_item_path = None
@@ -288,9 +291,16 @@ def get_parametrized_fixture_keys(
             else:
                 assert_never(scope)
 
-            yield FixtureArgKey(
-                argname, param_index, param_value, scoped_item_path, item_cls
-            )
+            param_index = cs.indices[argname]
+            param_value = cs.params[argname]
+            if isinstance(param_value, Hashable):
+                yield FixtureArgKeyByValue(
+                    argname, param_value, scoped_item_path, item_cls
+                )
+            else:
+                yield FixtureArgKeyByIndex(  # type: ignore[unreachable]
+                    argname, param_index, scoped_item_path, item_cls
+                )
 
 
 # Algorithm for sorting on a per-parametrized resource setup basis.

testing/python/fixtures.py

@@ -12,6 +12,7 @@ from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pytester import get_public_names
 from _pytest.pytester import Pytester
 from _pytest.python import Function
+from _pytest.scope import Scope
 
 
 def test_getfuncargnames_functions():
@@ -4683,3 +4684,54 @@ def test_reorder_with_high_scoped_direct_and_fixture_parametrization(
             r"  <Function test_3\[2\]>",
         ]
     )
+
+
+def test_get_parametrized_fixture_keys_with_unhashable_params(
+    pytester: Pytester,
+) -> None:
+    module = pytester.makepyfile(
+        """
+        import pytest
+
+        @pytest.mark.parametrize("arg", [[1], [2]], scope='module')
+        def test(arg):
+            pass
+    """
+    )
+    test_0, test_1 = pytester.genitems((pytester.getmodulecol(module),))
+    test_0_keys = list(fixtures.get_parametrized_fixture_keys(test_0, Scope.Module))
+    test_1_keys = list(fixtures.get_parametrized_fixture_keys(test_1, Scope.Module))
+    assert len(test_0_keys) == len(test_1_keys) == 1
+    assert isinstance(test_0_keys[0], fixtures.FixtureArgKeyByIndex)
+    assert test_0_keys[0].param_index == 0
+    assert isinstance(test_1_keys[0], fixtures.FixtureArgKeyByIndex)
+    assert test_1_keys[0].param_index == 1
+
+
+def test_reordering_with_unhashable_parametrize_args(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        import pytest
+
+        @pytest.mark.parametrize("arg", [[1], [2]], scope='module')
+        def test_1(arg):
+            print(arg)
+
+        def test_2():
+            print("test_2")
+
+        @pytest.mark.parametrize("arg", [[3], [4]], scope='module')
+        def test_3(arg):
+            print(arg)
+    """
+    )
+    result = pytester.runpytest("-s")
+    result.stdout.fnmatch_lines(
+        [
+            r"*1*",
+            r"*3*",
+            r"*2*",
+            r"*4*",
+            r"*test_2*",
+        ]
+    )