BUG: Fix metadata mutation in MultiIndex set operations

Author: deniskrds

pandas/core/indexes/multi.py

@@ -4082,13 +4082,12 @@ def _is_comparable_dtype(self, dtype: DtypeObj) -> bool:
     def _get_reconciled_name_object(self, other) -> MultiIndex:
         """
         If the result of a set operation will be self,
-        return self, unless the names change, in which
-        case make a shallow copy of self.
+        return a shallow copy of self.
         """
         names = self._maybe_match_names(other)
         if self.names != names:
             return self.rename(names)
-        return self
+        return self.copy()
 
     def _maybe_match_names(self, other):
         """

pandas/tests/indexes/test_setops.py

@@ -812,7 +812,7 @@ def test_union_identity(self, index, sort):
         first = index[5:20]
 
         union = first.union(first, sort=sort)
-        # GH#63169 - identity is never preserved now
+        # GH#63169 - identity is not preserved to prevent shared mutable state
         assert union is not first
 
         # This should no longer be the same object, since [] is not consistent,
@@ -1025,7 +1025,7 @@ def test_union_mutation_safety(self):
 
     def test_union_mutation_safety_other(self):
         # GH#63169
-        index1 = Index([], name="original")
+        index1 = Index([0, 1], name="original")
         index2 = Index([0, 1], name="original")
 
         result = index1.union(index2)
@@ -1039,3 +1039,25 @@ def test_union_mutation_safety_other(self):
 
         assert result.name == "original"
         assert index2.name == "changed"
+
+    def test_multiindex_intersection_mutation_safety(self):
+        # GH#63169
+        mi1 = MultiIndex.from_tuples([("a", 1), ("b", 2)], names=["x", "y"])
+        mi2 = MultiIndex.from_tuples([("a", 1), ("b", 2)], names=["x", "y"])
+
+        result = mi1.intersection(mi2)
+        assert result is not mi1
+
+        mi1.names = ["changed1", "changed2"]
+        assert result.names == ["x", "y"]
+
+    def test_multiindex_union_mutation_safety(self):
+        # GH#63169
+        mi1 = MultiIndex.from_tuples([("a", 1), ("b", 2)], names=["x", "y"])
+        mi2 = MultiIndex.from_tuples([("a", 1), ("b", 2)], names=["x", "y"])
+
+        result = mi1.union(mi2)
+        assert result is not mi1
+
+        mi1.names = ["changed1", "changed2"]
+        assert result.names == ["x", "y"]

pandas/tests/indexes/timedeltas/test_setops.py

@@ -114,7 +114,7 @@ def test_intersection_bug_1708(self):
 
     def test_intersection_equal(self, sort):
         # GH 24471 Test intersection outcome given the sort keyword
-        # for equal indices intersection should return the original index
+        # GH#63169 intersection returns a copy to prevent shared mutable state
         first = timedelta_range("1 day", periods=4, freq="h")
         second = timedelta_range("1 day", periods=4, freq="h")
         intersect = first.intersection(second, sort=sort)
@@ -124,7 +124,7 @@ def test_intersection_equal(self, sort):
 
         # Corner cases
         inter = first.intersection(first, sort=sort)
-        assert inter is first
+        assert inter is not first
 
     @pytest.mark.parametrize("period_1, period_2", [(0, 4), (4, 0)])
     def test_intersection_zero_length(self, period_1, period_2, sort):