BUG/API: Raise when extension class passed to astype (pandas-dev#17796)

Closes pandas-dev#17780

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -768,6 +768,7 @@ Conversion
 - Bug in :attr:`Timestamp.weekday_name` returning a UTC-based weekday name when localized to a timezone (:issue:`17354`)
 - Bug in ``Timestamp.replace`` when replacing ``tzinfo`` around DST changes (:issue:`15683`)
 - Bug in ``Timedelta`` construction and arithmetic that would not propagate the ``Overflow`` exception (:issue:`17367`)
+- Bug in :meth:`~DataFrame.astype` converting to object dtype when passeed extension type classes (`DatetimeTZDtype``, ``CategoricalDtype``) rather than instances. Now a ``TypeError`` is raised when a class is passed (:issue:`17780`).
 
 Indexing
 ^^^^^^^^

pandas/core/internals.py

@@ -1,6 +1,7 @@
 import warnings
 import copy
 from warnings import catch_warnings
+import inspect
 import itertools
 import re
 import operator
@@ -552,6 +553,11 @@ def _astype(self, dtype, copy=False, errors='raise', values=None,
                                list(errors_legal_values), errors))
             raise ValueError(invalid_arg)
 
+        if inspect.isclass(dtype) and issubclass(dtype, ExtensionDtype):
+            msg = ("Expected an instance of {}, but got the class instead. "
+                   "Try instantiating 'dtype'.".format(dtype.__name__))
+            raise TypeError(msg)
+
         # may need to convert to categorical
         # this is only called for non-categoricals
         if self.is_categorical_astype(dtype):

pandas/tests/frame/test_dtypes.py

@@ -612,6 +612,20 @@ def test_astype_duplicate_col(self):
         expected = concat([a1_str, b, a2_str], axis=1)
         assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("cls", [
+        pd.api.types.CategoricalDtype,
+        pd.api.types.DatetimeTZDtype,
+        pd.api.types.IntervalDtype
+    ])
+    def test_astype_categoricaldtype_class_raises(self, cls):
+        df = DataFrame({"A": ['a', 'a', 'b', 'c']})
+        xpr = "Expected an instance of {}".format(cls.__name__)
+        with tm.assert_raises_regex(TypeError, xpr):
+            df.astype({"A": cls})
+
+        with tm.assert_raises_regex(TypeError, xpr):
+            df['A'].astype(cls)
+
     def test_timedeltas(self):
         df = DataFrame(dict(A=Series(date_range('2012-1-1', periods=3,
                                                 freq='D')),