TST/DOC: Fix tests and docs for .cat raising AttributeError if invalid

Follow-up on GH9617

Fixes GH8814 -- is it fair to say that ``hasattr(s, 'cat')`` is probably
the best solution we're going to come up with for checking for categorical
data?

Author: shoyer

doc/source/categorical.rst

@@ -766,6 +766,14 @@ Dtype comparisons work:
     dtype == np.str_
     np.str_ == dtype
 
+To check if a Series contains Categorical data, with pandas 0.16 or later, use
+``hasattr(s, 'cat')``:
+
+.. ipython:: python
+
+    hasattr(Series(['a'], dtype='category'), 'cat')
+    hasattr(Series(['a']), 'cat')
+
 Using `numpy` functions on a `Series` of type ``category`` should not work as `Categoricals`
 are not numeric data (even in the case that ``.categories`` is numeric).
 

pandas/core/generic.py

@@ -78,10 +78,11 @@ class NDFrame(PandasObject):
     copy : boolean, default False
     """
     _internal_names = ['_data', '_cacher', '_item_cache', '_cache',
-                       'is_copy', 'dt', 'cat', 'str', '_subtyp', '_index',
+                       'is_copy', '_subtyp', '_index',
                        '_default_kind', '_default_fill_value',
                        '__array_struct__','__array_interface__']
     _internal_names_set = set(_internal_names)
+    _accessors = frozenset([])
     _metadata = []
     is_copy = None
 
@@ -1957,9 +1958,9 @@ def __getattr__(self, name):
         # Note: obj.x will always call obj.__getattribute__('x') prior to
         # calling obj.__getattr__('x').
 
-        if name in self._internal_names_set:
-            return object.__getattribute__(self, name)
-        elif name in self._metadata:
+        if (name in self._internal_names_set
+                or name in self._metadata
+                or name in self._accessors):
             return object.__getattribute__(self, name)
         else:
             if name in self._info_axis:

pandas/core/series.py

@@ -113,6 +113,7 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
         Copy input data
     """
     _metadata = ['name']
+    _accessors = frozenset(['dt', 'cat', 'str'])
     _allow_index_ops = True
 
     def __init__(self, data=None, index=None, dtype=None, name=None,

pandas/tests/test_categorical.py

@@ -1354,13 +1354,13 @@ def test_sequence_like(self):
     def test_series_delegations(self):
 
         # invalid accessor
-        self.assertRaises(TypeError, lambda : Series([1,2,3]).cat)
-        tm.assertRaisesRegexp(TypeError,
+        self.assertRaises(AttributeError, lambda : Series([1,2,3]).cat)
+        tm.assertRaisesRegexp(AttributeError,
                               r"Can only use .cat accessor with a 'category' dtype",
                               lambda : Series([1,2,3]).cat)
-        self.assertRaises(TypeError, lambda : Series(['a','b','c']).cat)
-        self.assertRaises(TypeError, lambda : Series(np.arange(5.)).cat)
-        self.assertRaises(TypeError, lambda : Series([Timestamp('20130101')]).cat)
+        self.assertRaises(AttributeError, lambda : Series(['a','b','c']).cat)
+        self.assertRaises(AttributeError, lambda : Series(np.arange(5.)).cat)
+        self.assertRaises(AttributeError, lambda : Series([Timestamp('20130101')]).cat)
 
         # Series should delegate calls to '.categories', '.codes', '.ordered' and the
         # methods '.set_categories()' 'drop_unused_categories()' to the categorical