BUG: Fix SparseFrameAccessor.to_dense return type

Author: janezd

doc/source/whatsnew/v3.0.0.rst

@@ -682,6 +682,7 @@ Sparse
 ^^^^^^
 - Bug in :class:`SparseDtype` for equal comparison with na fill value. (:issue:`54770`)
 - Bug in :meth:`DataFrame.sparse.from_spmatrix` which hard coded an invalid ``fill_value`` for certain subtypes. (:issue:`59063`)
+- Bug in :meth:`DataFrams.sparse.to_dense` which ignored subclassing and always returned an instance of :class:`DataFrame` (:issue:`59913`)
 
 ExtensionArray
 ^^^^^^^^^^^^^^

pandas/core/arrays/sparse/accessor.py

@@ -369,10 +369,9 @@ def to_dense(self) -> DataFrame:
         1  1
         2  0
         """
-        from pandas import DataFrame
-
+        constructor = self._parent._constructor
         data = {k: v.array.to_dense() for k, v in self._parent.items()}
-        return DataFrame(data, index=self._parent.index, columns=self._parent.columns)
+        return constructor(data, index=self._parent.index, columns=self._parent.columns)
 
     def to_coo(self) -> spmatrix:
         """

pandas/tests/arrays/sparse/test_accessor.py

@@ -252,3 +252,12 @@ def test_with_column_named_sparse(self):
         # https://github.com/pandas-dev/pandas/issues/30758
         df = pd.DataFrame({"sparse": pd.arrays.SparseArray([1, 2])})
         assert isinstance(df.sparse, pd.core.arrays.sparse.accessor.SparseFrameAccessor)
+
+    def test_subclassing(self):
+        class SubclassedDataFrame(pd.DataFrame):
+            @property
+            def _constructor(self):
+                return SubclassedDataFrame
+
+        df = SubclassedDataFrame({"sparse": pd.arrays.SparseArray([1, 2])})
+        assert isinstance(df.sparse.to_dense(), SubclassedDataFrame)