BUG: astype fill_value for SparseArray.astype

pandas/core/arrays/sparse.py

@@ -284,6 +284,83 @@ def is_dtype(cls, dtype):
             return True
         return isinstance(dtype, np.dtype) or dtype == 'Sparse'
 
+    def astype(self, dtype):
+        """Convert the SparseDtype to a new dtype.
+
+        This takes care of converting the ``fill_value``.
+
+        Parameters
+        ----------
+        dtype : Union[str, numpy.dtype, SparseDtype]
+            The new dtype to use.
+
+            * For a SparseDtype, it is simply returned
+            * For a NumPy dtype (or str), the current fill value
+              is converted to the new dtype, and a SparseDtype
+              with `dtype` and the new fill value is returned.
+
+        Returns
+        -------
+        SparseDtype
+            A new SparseDtype with the corret `dtype` and fill value
+            for that `dtype`.
+
+        Raises
+        ------
+        ValueError
+            When the current fill value cannot be converted to the
+            new `dtype` (e.g. trying to convert ``np.nan`` to an
+            integer dtype).
+
+
+        Examples
+        --------
+        >>> SparseDtype(int, 0).astype(float)
+        Sparse[float64, 0.0]
+
+        >>> SparseDtype(int, 1).astype(SparseDtype(float, np.nan))
+        Sparse[float64, nan]
+        """
+        cls = type(self)
+        dtype = pandas_dtype(dtype)
+
+        if not isinstance(dtype, cls):
+            fill_value = astype_nansafe(np.array(self.fill_value),
+                                        dtype).item()
+            dtype = cls(dtype, fill_value=fill_value)
+
+        return dtype
+
+    @property
+    def _subtype_with_str(self):
+        """
+        Whether the SparseDtype's subtype should be considered ``str``.
+
+        Typically, pandas will store string data in an object-dtype array.
+        When converting values to a dtype, e.g. in ``.astype``, we need to
+        be more specific, we need the actual underlying type.
+
+        Returns
+        -------
+
+        >>> SparseDtype(int, 1)._subtype_with_str
+        dtype('int64')
+
+        >>> SparseDtype(object, 1)._subtype_with_str
+        dtype('O')
+
+        >>> dtype = SparseDtype(str, '')
+        >>> dtype.subtype
+        dtype('O')
+
+        >>> dtype._subtype_with_str
+        str
+        """
+        if isinstance(self.fill_value, compat.string_types):
+            return type(self.fill_value)
+        return self.subtype
+
+
 # ----------------------------------------------------------------------------
 # Array
 
@@ -614,7 +691,7 @@ def __array__(self, dtype=None, copy=True):
                     # Can't put pd.NaT in a datetime64[ns]
                     fill_value = np.datetime64('NaT')
             try:
-                dtype = np.result_type(self.sp_values.dtype, fill_value)
+                dtype = np.result_type(self.sp_values.dtype, type(fill_value))
             except TypeError:
                 dtype = object
 
@@ -996,7 +1073,7 @@ def _take_with_fill(self, indices, fill_value=None):
         if len(self) == 0:
             # Empty... Allow taking only if all empty
             if (indices == -1).all():
-                dtype = np.result_type(self.sp_values, fill_value)
+                dtype = np.result_type(self.sp_values, type(fill_value))
                 taken = np.empty_like(indices, dtype=dtype)
                 taken.fill(fill_value)
                 return taken
@@ -1009,7 +1086,7 @@ def _take_with_fill(self, indices, fill_value=None):
         if self.sp_index.npoints == 0:
             # Avoid taking from the empty self.sp_values
             taken = np.full(sp_indexer.shape, fill_value=fill_value,
-                            dtype=np.result_type(fill_value))
+                            dtype=np.result_type(type(fill_value)))
         else:
             taken = self.sp_values.take(sp_indexer)
 
@@ -1030,12 +1107,13 @@ def _take_with_fill(self, indices, fill_value=None):
             result_type = taken.dtype
 
             if m0.any():
-                result_type = np.result_type(result_type, self.fill_value)
+                result_type = np.result_type(result_type,
+                                             type(self.fill_value))
                 taken = taken.astype(result_type)
                 taken[old_fill_indices] = self.fill_value
 
             if m1.any():
-                result_type = np.result_type(result_type, fill_value)
+                result_type = np.result_type(result_type, type(fill_value))
                 taken = taken.astype(result_type)
                 taken[new_fill_indices] = fill_value
 
@@ -1061,7 +1139,7 @@ def _take_without_fill(self, indices):
             # edge case in take...
             # I think just return
             out = np.full(indices.shape, self.fill_value,
-                          dtype=np.result_type(self.fill_value))
+                          dtype=np.result_type(type(self.fill_value)))
             arr, sp_index, fill_value = make_sparse(out,
                                                     fill_value=self.fill_value)
             return type(self)(arr, sparse_index=sp_index,
@@ -1073,7 +1151,7 @@ def _take_without_fill(self, indices):
 
         if fillable.any():
             # TODO: may need to coerce array to fill value
-            result_type = np.result_type(taken, self.fill_value)
+            result_type = np.result_type(taken, type(self.fill_value))
             taken = taken.astype(result_type)
             taken[fillable] = self.fill_value
 
@@ -1212,13 +1290,10 @@ def astype(self, dtype=None, copy=True):
         IntIndex
         Indices: array([2, 3], dtype=int32)
         """
-        dtype = pandas_dtype(dtype)
-
-        if not isinstance(dtype, SparseDtype):
-            dtype = SparseDtype(dtype, fill_value=self.fill_value)
-
+        dtype = self.dtype.astype(dtype)
+        subtype = dtype._subtype_with_str
         sp_values = astype_nansafe(self.sp_values,
-                                   dtype.subtype,
+                                   subtype,
                                    copy=copy)
         if sp_values is self.sp_values and copy:
             sp_values = sp_values.copy()

pandas/tests/arrays/sparse/test_array.py

@@ -477,6 +477,34 @@ def test_astype_all(self, any_real_dtype):
         tm.assert_numpy_array_equal(np.asarray(res.values),
                                     vals.astype(typ))
 
+    @pytest.mark.parametrize('array, dtype, expected', [
+        (SparseArray([0, 1]), 'float',
+         SparseArray([0., 1.], dtype=SparseDtype(float, 0.0))),
+        (SparseArray([0, 1]), bool, SparseArray([False, True])),
+        (SparseArray([0, 1], fill_value=1), bool,
+         SparseArray([False, True], dtype=SparseDtype(bool, True))),
+        pytest.param(
+            SparseArray([0, 1]), 'datetime64[ns]',
+            SparseArray(np.array([0, 1], dtype='datetime64[ns]'),
+                        dtype=SparseDtype('datetime64[ns]',
+                                          pd.Timestamp('1970'))),
+            marks=[pytest.mark.xfail(reason="NumPy-7619", strict=True)],
+        ),
+        (SparseArray([0, 1, 10]), str,
+         SparseArray(['0', '1', '10'], dtype=SparseDtype(str, '0'))),
+        (SparseArray(['10', '20']), float, SparseArray([10.0, 20.0])),
+        (SparseArray([0, 1, 0]), object,
+         SparseArray([0, 1, 0], dtype=SparseDtype(object, 0))),
+    ])
+    def test_astype_more(self, array, dtype, expected):
+        result = array.astype(dtype)
+        tm.assert_sp_array_equal(result, expected)
+
+    def test_astype_nan_raises(self):
+        arr = SparseArray([1.0, np.nan])
+        with tm.assert_raises_regex(ValueError, 'Cannot convert non-finite'):
+            arr.astype(int)
+
     def test_set_fill_value(self):
         arr = SparseArray([1., np.nan, 2.], fill_value=np.nan)
         arr.fill_value = 2

pandas/tests/arrays/sparse/test_dtype.py

@@ -139,3 +139,23 @@ def test_parse_subtype(string, expected):
 def test_construct_from_string_fill_value_raises(string):
     with pytest.raises(TypeError, match='fill_value in the string is not'):
         SparseDtype.construct_from_string(string)
+
+
+@pytest.mark.parametrize('original, dtype, expected', [
+    (SparseDtype(int, 0), float, SparseDtype(float, 0.0)),
+    (SparseDtype(int, 1), float, SparseDtype(float, 1.0)),
+    (SparseDtype(int, 1), str, SparseDtype(object, '1')),
+    (SparseDtype(float, 1.5), int, SparseDtype(int, 1)),
+])
+def test_astype(original, dtype, expected):
+    result = original.astype(dtype)
+    assert result == expected
+
+
+@pytest.mark.parametrize("original, dtype", [
+    (SparseDtype(float, np.nan), int),
+    (SparseDtype(str, 'abc'), int),
+])
+def test_astype_raises(original, dtype):
+    with pytest.raises(ValueError):
+        original.astype(dtype)