cleaner way to get ndarray from DTI and fix failing pytables tests

Author: reidy-p

pandas/core/indexes/base.py

@@ -506,9 +506,9 @@ def _shallow_copy(self, values=None, **kwargs):
         attributes.update(kwargs)
         if not len(values) and 'dtype' not in kwargs:
             attributes['dtype'] = self.dtype
-        from pandas import DatetimeIndex
-        if isinstance(values, DatetimeIndex):
-            values = values.values
+
+        # _simple_new expects an ndarray
+        values = getattr(values, 'values', values)
         return self._simple_new(values, **attributes)
 
     def _shallow_copy_with_infer(self, values=None, **kwargs):

pandas/core/indexes/datetimes.py

@@ -612,6 +612,7 @@ def _simple_new(cls, values, name=None, freq=None, tz=None,
                            dtype=dtype, **kwargs)
             values = np.array(values, copy=False)
 
+        # values should be a numpy array
         assert isinstance(values, np.ndarray)
 
         if is_object_dtype(values):

pandas/io/pytables.py

@@ -2472,7 +2472,8 @@ def _get_index_factory(self, klass):
         if klass == DatetimeIndex:
             def f(values, freq=None, tz=None):
                 # data are already in UTC, localize and convert if tz present
-                result = DatetimeIndex._simple_new(values, None, freq=freq)
+                result = DatetimeIndex._simple_new(values.values, None,
+                                                   freq=freq)
                 if tz is not None:
                     result = result.tz_localize('UTC').tz_convert(tz)
                 return result