Backport PR #42823: BUG: Pass copy argument to expanddim constructor in concat. (#42984)

Co-authored-by: Justin McOmie <justin.mcomie@gmail.com>

Author: meeseeksmachine

doc/source/whatsnew/v1.3.2.rst

@@ -23,6 +23,7 @@ Fixed regressions
 - Fixed regression where :meth:`pandas.read_csv` raised a ``ValueError`` when parameters ``names`` and ``prefix`` were both set to None (:issue:`42387`)
 - Fixed regression in comparisons between :class:`Timestamp` object and ``datetime64`` objects outside the implementation bounds for nanosecond ``datetime64`` (:issue:`42794`)
 - Fixed regression in :meth:`.Styler.highlight_min` and :meth:`.Styler.highlight_max` where ``pandas.NA`` was not successfully ignored (:issue:`42650`)
+- Fixed regression in :func:`pandas.concat` where ``copy=False`` was not honored in ``axis=1`` Series concatenation (:issue:`42501`)
 - Regression in :meth:`Series.nlargest` and :meth:`Series.nsmallest` with nullable integer or float dtype (:issue:`41816`)
 - Fixed regression in :meth:`pandas.Series.quantile` with :class:`pandas.Int64Dtype` (:issue:`42626`)
 

pandas/core/reshape/concat.py

@@ -505,7 +505,7 @@ def get_result(self):
                 cons = sample._constructor_expanddim
 
                 index, columns = self.new_axes
-                df = cons(data, index=index)
+                df = cons(data, index=index, copy=self.copy)
                 df.columns = columns
                 return df.__finalize__(self, method="concat")
 

pandas/tests/extension/decimal/test_decimal.py

@@ -261,18 +261,7 @@ def test_dataframe_constructor_with_dtype():
     tm.assert_frame_equal(result, expected)
 
 
-@pytest.mark.parametrize(
-    "frame",
-    [
-        pytest.param(
-            True,
-            marks=pytest.mark.xfail(
-                reason="pd.concat call inside NDFrame.astype reverts the dtype"
-            ),
-        ),
-        False,
-    ],
-)
+@pytest.mark.parametrize("frame", [True, False])
 def test_astype_dispatches(frame):
     # This is a dtype-specific test that ensures Series[decimal].astype
     # gets all the way through to ExtensionArray.astype

pandas/tests/frame/methods/test_astype.py

@@ -23,6 +23,7 @@
     option_context,
 )
 import pandas._testing as tm
+from pandas.core.arrays.integer import coerce_to_array
 
 
 def _check_cast(df, v):
@@ -726,3 +727,32 @@ def test_astype_categorical_to_string_missing(self):
         cat = df.astype("category")
         result = cat.astype(str)
         tm.assert_frame_equal(result, expected)
+
+
+class IntegerArrayNoCopy(pd.core.arrays.IntegerArray):
+    # GH 42501
+
+    @classmethod
+    def _from_sequence(cls, scalars, *, dtype=None, copy=False):
+        values, mask = coerce_to_array(scalars, dtype=dtype, copy=copy)
+        return IntegerArrayNoCopy(values, mask)
+
+    def copy(self):
+        assert False
+
+
+class Int16DtypeNoCopy(pd.Int16Dtype):
+    # GH 42501
+
+    @classmethod
+    def construct_array_type(cls):
+        return IntegerArrayNoCopy
+
+
+def test_frame_astype_no_copy():
+    # GH 42501
+    df = DataFrame({"a": [1, 4, None, 5], "b": [6, 7, 8, 9]}, dtype=object)
+    result = df.astype({"a": Int16DtypeNoCopy()}, copy=False)
+
+    assert result.a.dtype == pd.Int16Dtype()
+    assert np.shares_memory(df.b.values, result.b.values)