Make Series works the same as dict in pd.astype()

doc/source/whatsnew/v0.21.0.txt

@@ -94,6 +94,8 @@ Bug Fixes
 
 Conversion
 ^^^^^^^^^^
+- Fix a bug when pd.astype() receive Series as `dtype` paramter, no action is taken. Now Series 
+`dtype` work the same as a `dict`.
 
 
 

pandas/core/generic.py

@@ -3507,7 +3507,7 @@ def astype(self, dtype, copy=True, errors='raise', **kwargs):
         -------
         casted : type of caller
         """
-        if isinstance(dtype, collections.Mapping):
+        if is_dict_like(dtype):
             if self.ndim == 1:  # i.e. Series
                 if len(dtype) > 1 or list(dtype.keys())[0] != self.name:
                     raise KeyError('Only the Series name can be used for '

pandas/tests/frame/test_dtypes.py

@@ -487,6 +487,54 @@ def test_astype_dict(self):
         assert_frame_equal(df, equiv)
         assert_frame_equal(df, original)
 
+    def test_astype_Series(self):
+        # GH16717
+        a = Series(date_range('2010-01-04', periods=5))
+        b = Series(range(5))
+        c = Series([0.0, 0.2, 0.4, 0.6, 0.8])
+        d = Series(['1.0', '2', '3.14', '4', '5.4'])
+        df = DataFrame({'a': a, 'b': b, 'c': c, 'd': d})
+        original = df.copy(deep=True)
+
+        # change type of a subset of columns
+        result = df.astype(Series({'b': 'str', 'd': 'float32'}))
+        expected = DataFrame({
+            'a': a,
+            'b': Series(['0', '1', '2', '3', '4']),
+            'c': c,
+            'd': Series([1.0, 2.0, 3.14, 4.0, 5.4], dtype='float32')})
+        assert_frame_equal(result, expected)
+        assert_frame_equal(df, original)
+
+        result = df.astype(Series(
+            {'b': np.float32,
+             'c': 'float32',
+             'd': np.float64}))
+        expected = DataFrame({
+            'a': a,
+            'b': Series([0.0, 1.0, 2.0, 3.0, 4.0], dtype='float32'),
+            'c': Series([0.0, 0.2, 0.4, 0.6, 0.8], dtype='float32'),
+            'd': Series([1.0, 2.0, 3.14, 4.0, 5.4], dtype='float64')})
+        assert_frame_equal(result, expected)
+        assert_frame_equal(df, original)
+
+        # change all columns
+        result = df.astype(Series({'a': str, 'b': str, 'c': str, 'd': str}))
+        assert_frame_equal(result, df.astype(str))
+        assert_frame_equal(df, original)
+
+        # error should be raised when using something other than column labels
+        # in the keys of the dtype dict
+        pytest.raises(KeyError, df.astype, Series({'b': str, 2: str}))
+        pytest.raises(KeyError, df.astype, Series({'e': str}))
+        assert_frame_equal(df, original)
+
+        # if the dtypes provided are the same as the original dtypes, the
+        # resulting DataFrame should be the same as the original DataFrame
+        equiv = df.astype(Series({col: df[col].dtype for col in df.columns}))
+        assert_frame_equal(df, equiv)
+        assert_frame_equal(df, original)
+
     def test_astype_duplicate_col(self):
         a1 = Series([1, 2, 3, 4, 5], name='a')
         b = Series([0.1, 0.2, 0.4, 0.6, 0.8], name='b')