BUG: Pass copy argument to expanddim constructor in concat.

Author: jmcomie

pandas/core/reshape/concat.py

@@ -504,7 +504,7 @@ 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 +727,29 @@ 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({"col": [1, 4, None, 5]}, dtype=object)
+    df = df.astype({"col": Int16DtypeNoCopy()}, copy=False)