BUG: DataFrame.astype preserve subclasses GH#40810 (#45272)

Author: jbrockmendel

doc/source/whatsnew/v1.5.0.rst

@@ -184,6 +184,7 @@ Numeric
 
 Conversion
 ^^^^^^^^^^
+- Bug in :meth:`DataFrame.astype` not preserving subclasses (:issue:`40810`)
 - Bug in constructing a :class:`Series` from a float-containing list or a floating-dtype ndarray-like (e.g. ``dask.Array``) and an integer dtype raising instead of casting like we would with an ``np.ndarray`` (:issue:`40110`)
 -
 

pandas/core/generic.py

@@ -5896,6 +5896,12 @@ def astype(
 
         # GH 19920: retain column metadata after concat
         result = concat(results, axis=1, copy=False)
+        # GH#40810 retain subclass
+        # Incompatible types in assignment (expression has type "NDFrameT",
+        # variable has type "DataFrame")
+        # Argument 1 to "NDFrame" has incompatible type "DataFrame"; expected
+        # "Union[ArrayManager, SingleArrayManager, BlockManager, SingleBlockManager]"
+        result = self._constructor(result)  # type: ignore[arg-type,assignment]
         result.columns = self.columns
         result = result.__finalize__(self, method="astype")
         # https://github.com/python/mypy/issues/8354

pandas/tests/frame/test_subclass.py

@@ -723,6 +723,13 @@ def test_convert_dtypes_preserves_subclass(self, gpd_style_subclass_df):
         result = gpd_style_subclass_df.convert_dtypes()
         assert isinstance(result, type(gpd_style_subclass_df))
 
+    def test_astype_preserves_subclass(self):
+        # GH#40810
+        df = tm.SubclassedDataFrame({"A": [1, 2, 3], "B": [4, 5, 6], "C": [7, 8, 9]})
+
+        result = df.astype({"A": np.int64, "B": np.int32, "C": np.float64})
+        assert isinstance(result, tm.SubclassedDataFrame)
+
     def test_equals_subclass(self):
         # https://github.com/pandas-dev/pandas/pull/34402
         # allow subclass in both directions