sparse

Author: TomAugspurger

pandas/core/internals/blocks.py

@@ -36,6 +36,7 @@
     is_list_like,
     is_re,
     is_re_compilable,
+    is_sparse,
     pandas_dtype)
 from pandas.core.dtypes.cast import (
     maybe_downcast_to_dtype,
@@ -633,7 +634,9 @@ def _astype(self, dtype, copy=False, errors='raise', values=None,
             return self
 
         if klass is None:
-            if dtype == np.object_:
+            # sparse is "special" and preserves sparsity.
+            # We're changing this in GH-23125
+            if dtype == np.object_ and is_sparse(values):
                 klass = ObjectBlock
             elif is_extension_array_dtype(dtype):
                 klass = ExtensionBlock

pandas/tests/sparse/frame/test_frame.py

@@ -736,6 +736,16 @@ def test_astype_bool(self):
         assert res['A'].dtype == SparseDtype(np.bool)
         assert res['B'].dtype == SparseDtype(np.bool)
 
+    def test_astype_object(self):
+        # This may change in GH-23125
+        df = pd.DataFrame({"A": SparseArray([0, 1]),
+                           "B": SparseArray([0, 1])})
+        result = df.astype(object)
+        dtype = SparseDtype(object, 0)
+        expected = pd.DataFrame({"A": SparseArray([0, 1], dtype=dtype),
+                                 "B": SparseArray([0, 1], dtype=dtype)})
+        tm.assert_frame_equal(result, expected)
+
     def test_fillna(self, float_frame_fill0, float_frame_fill0_dense):
         df = float_frame_fill0.reindex(lrange(5))
         dense = float_frame_fill0_dense.reindex(lrange(5))