ENH: Add lazy copy to swaplevel

pandas/core/frame.py

@@ -7254,7 +7254,7 @@ def nsmallest(self, n: int, columns: IndexLabel, keep: str = "first") -> DataFra
         ),
     )
     def swaplevel(self, i: Axis = -2, j: Axis = -1, axis: Axis = 0) -> DataFrame:
-        result = self.copy()
+        result = self.copy(deep=None)
 
         axis = self._get_axis_number(axis)
 

pandas/core/series.py

@@ -4071,7 +4071,9 @@ def nsmallest(self, n: int = 5, keep: str = "first") -> Series:
         dtype: object"""
         ),
     )
-    def swaplevel(self, i: Level = -2, j: Level = -1, copy: bool = True) -> Series:
+    def swaplevel(
+        self, i: Level = -2, j: Level = -1, copy: bool | None = None
+    ) -> Series:
         """
         Swap levels i and j in a :class:`MultiIndex`.
 
@@ -4091,10 +4093,9 @@ def swaplevel(self, i: Level = -2, j: Level = -1, copy: bool = True) -> Series:
         {examples}
         """
         assert isinstance(self.index, MultiIndex)
-        new_index = self.index.swaplevel(i, j)
-        return self._constructor(self._values, index=new_index, copy=copy).__finalize__(
-            self, method="swaplevel"
-        )
+        result = self.copy(deep=copy)
+        result.index = self.index.swaplevel(i, j)
+        return result
 
     def reorder_levels(self, order: Sequence[Level]) -> Series:
         """

pandas/tests/copy_view/test_methods.py

@@ -424,6 +424,24 @@ def test_reorder_levels(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+@pytest.mark.parametrize("obj", [Series([1, 2, 3]), DataFrame({"a": [1, 2, 3]})])
+def test_swaplevel(using_copy_on_write, obj):
+    index = MultiIndex.from_tuples([(1, 1), (1, 2), (2, 1)], names=["one", "two"])
+    obj.index = index
+    obj_orig = obj.copy()
+    obj2 = obj.swaplevel()
+
+    if using_copy_on_write:
+        assert np.shares_memory(obj2.values, obj.values)
+    else:
+        assert not np.shares_memory(obj2.values, obj.values)
+
+    obj2.iloc[0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(obj2.values, obj.values)
+    tm.assert_equal(obj, obj_orig)
+
+
 def test_frame_set_axis(using_copy_on_write):
     # GH 49473
     df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [0.1, 0.2, 0.3]})