BUG: transpose not respecting CoW

doc/source/whatsnew/v2.0.0.rst

@@ -261,6 +261,8 @@ Copy-on-Write improvements
 - :meth:`DataFrame.replace` will now respect the Copy-on-Write mechanism
   when ``inplace=True``.
 
+- :meth:`DataFrame.transpose` will now respect the Copy-on-Write mechanism.
+
 - Arithmetic operations that can be inplace, e.g. ``ser *= 2`` will now respect the
   Copy-on-Write mechanism.
 

pandas/core/frame.py

@@ -3557,10 +3557,14 @@ def transpose(self, *args, copy: bool = False) -> DataFrame:
         if self._can_fast_transpose:
             # Note: tests pass without this, but this improves perf quite a bit.
             new_vals = self._values.T
-            if copy:
+            if copy and not using_copy_on_write():
                 new_vals = new_vals.copy()
 
-            result = self._constructor(new_vals, index=self.columns, columns=self.index)
+            result = self._constructor(
+                new_vals, index=self.columns, columns=self.index, copy=False
+            )
+            if using_copy_on_write() and len(self) > 0:
+                result._mgr.add_references(self._mgr)  # type: ignore[arg-type]
 
         elif (
             self._is_homogeneous_type and dtypes and is_extension_array_dtype(dtypes[0])

pandas/tests/copy_view/test_methods.py

@@ -1593,3 +1593,37 @@ def test_inplace_arithmetic_series_with_reference(using_copy_on_write):
         tm.assert_series_equal(ser_orig, view)
     else:
         assert np.shares_memory(get_array(ser), get_array(view))
+
+
+@pytest.mark.parametrize("copy", [True, False])
+def test_transpose(using_copy_on_write, copy):
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
+    df_orig = df.copy()
+    result = df.transpose(copy=copy)
+
+    if not copy or using_copy_on_write:
+        assert np.shares_memory(get_array(df, "a"), get_array(result, 0))
+    else:
+        assert not np.shares_memory(get_array(df, "a"), get_array(result, 0))
+
+    result.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_frame_equal(df, df_orig)
+
+
+def test_transpose_different_dtypes(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": 1.5})
+    df_orig = df.copy()
+    result = df.T
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(result, 0))
+    result.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_frame_equal(df, df_orig)
+
+
+def test_transpose_ea_single_column(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]}, dtype="Int64")
+    result = df.T
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(result, 0))