ENH: test CoW for drop_level

pandas/core/generic.py

@@ -853,7 +853,7 @@ def droplevel(self: NDFrameT, level: IndexLabel, axis: Axis = 0) -> NDFrameT:
         """
         labels = self._get_axis(axis)
         new_labels = labels.droplevel(level)
-        return self.set_axis(new_labels, axis=axis)
+        return self.set_axis(new_labels, axis=axis, copy=None)
 
     def pop(self, item: Hashable) -> Series | Any:
         result = self[item]

pandas/tests/copy_view/test_methods.py

@@ -594,3 +594,22 @@ def test_tz_convert_localize(using_copy_on_write, func, tz):
     ser2.iloc[0] = 0
     assert not np.shares_memory(ser2.values, ser.values)
     tm.assert_series_equal(ser, ser_orig)
+
+
+def test_droplevel(using_copy_on_write):
+    # GH 49473
+    index = MultiIndex.from_tuples([(1, 1), (1, 2), (2, 1)], names=["one", "two"])
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]}, index=index)
+    df_orig = df.copy()
+    df2 = df.droplevel(0)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+    else:
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+
+    # mutating df2 triggers a copy-on-write for that column / block
+    df2.iloc[0, 0] = 0
+
+    assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+    tm.assert_frame_equal(df, df_orig)