ENH: Add lazy copy for series.reorder_levels (#50472)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: phofl

pandas/core/series.py

@@ -4114,7 +4114,7 @@ def reorder_levels(self, order: Sequence[Level]) -> Series:
         if not isinstance(self.index, MultiIndex):  # pragma: no cover
             raise Exception("Can only reorder levels on a hierarchical axis.")
 
-        result = self.copy()
+        result = self.copy(deep=None)
         assert isinstance(result.index, MultiIndex)
         result.index = result.index.reorder_levels(order)
         return result

pandas/tests/copy_view/test_methods.py

@@ -443,6 +443,25 @@ def test_reorder_levels(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+def test_series_reorder_levels(using_copy_on_write):
+    index = MultiIndex.from_tuples(
+        [(1, 1), (1, 2), (2, 1), (2, 2)], names=["one", "two"]
+    )
+    ser = Series([1, 2, 3, 4], index=index)
+    ser_orig = ser.copy()
+    ser2 = ser.reorder_levels(order=["two", "one"])
+
+    if using_copy_on_write:
+        assert np.shares_memory(ser2.values, ser.values)
+    else:
+        assert not np.shares_memory(ser2.values, ser.values)
+
+    ser2.iloc[0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(ser2.values, ser.values)
+    tm.assert_series_equal(ser, ser_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"])
@@ -478,28 +497,6 @@ def test_frame_set_axis(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
-@pytest.mark.parametrize(
-    "func, tz", [("tz_convert", "Europe/Berlin"), ("tz_localize", None)]
-)
-def test_tz_convert_localize(using_copy_on_write, func, tz):
-    # GH 49473
-    ser = Series(
-        [1, 2], index=date_range(start="2014-08-01 09:00", freq="H", periods=2, tz=tz)
-    )
-    ser_orig = ser.copy()
-    ser2 = getattr(ser, func)("US/Central")
-
-    if using_copy_on_write:
-        assert np.shares_memory(ser.values, ser2.values)
-    else:
-        assert not np.shares_memory(ser.values, ser2.values)
-
-    # mutating ser triggers a copy-on-write for the column / block
-    ser2.iloc[0] = 0
-    assert not np.shares_memory(ser2.values, ser.values)
-    tm.assert_series_equal(ser, ser_orig)
-
-
 def test_series_set_axis(using_copy_on_write):
     # GH 49473
     ser = Series([1, 2, 3])
@@ -533,3 +530,25 @@ def test_rename_axis(using_copy_on_write, kwargs, copy_kwargs):
     if using_copy_on_write:
         assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
     tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize(
+    "func, tz", [("tz_convert", "Europe/Berlin"), ("tz_localize", None)]
+)
+def test_tz_convert_localize(using_copy_on_write, func, tz):
+    # GH 49473
+    ser = Series(
+        [1, 2], index=date_range(start="2014-08-01 09:00", freq="H", periods=2, tz=tz)
+    )
+    ser_orig = ser.copy()
+    ser2 = getattr(ser, func)("US/Central")
+
+    if using_copy_on_write:
+        assert np.shares_memory(ser.values, ser2.values)
+    else:
+        assert not np.shares_memory(ser.values, ser2.values)
+
+    # mutating ser triggers a copy-on-write for the column / block
+    ser2.iloc[0] = 0
+    assert not np.shares_memory(ser2.values, ser.values)
+    tm.assert_series_equal(ser, ser_orig)