BUG-20629 allow .at accessor with CategoricalIndex

doc/source/whatsnew/v0.25.0.rst

@@ -262,7 +262,7 @@ Bug Fixes
 Categorical
 ^^^^^^^^^^^
 
--
+- Bug in :func:`DataFrame.at` and :func:`Series.at` that would raise exception if the index was a :class:`CategoricalIndex` (:issue:`20629`)
 -
 -
 

pandas/core/frame.py

@@ -2710,13 +2710,19 @@ def _get_value(self, index, col, takeable=False):
 
         try:
             return engine.get_value(series._values, index)
+        except KeyError as e:
+            if self.index.nlevels > 1:
+                raise e  # partial indexing forbidden
+            else:
+                pass  # GH 20629
         except (TypeError, ValueError):
+            pass
 
-            # we cannot handle direct indexing
-            # use positional
-            col = self.columns.get_loc(col)
-            index = self.index.get_loc(index)
-            return self._get_value(index, col, takeable=True)
+        # we cannot handle direct indexing
+        # use positional
+        col = self.columns.get_loc(col)
+        index = self.index.get_loc(index)
+        return self._get_value(index, col, takeable=True)
     _get_value.__doc__ = get_value.__doc__
 
     def set_value(self, index, col, value, takeable=False):

pandas/core/indexes/category.py

@@ -498,6 +498,8 @@ def get_value(self, series, key):
             k = self._convert_scalar_indexer(k, kind='getitem')
             indexer = self.get_loc(k)
             return series.iloc[indexer]
+        except AttributeError:
+            return super().get_value(series, indexer)
         except (KeyError, TypeError):
             pass
 

pandas/tests/indexing/test_categorical.py

@@ -719,3 +719,11 @@ def test_map_with_dict_or_series(self):
         output = cur_index.map(mapper)
         # Order of categories in output can be different
         tm.assert_index_equal(expected, output)
+
+    def test_at_with_categorical_index(self):
+        # GH 20629
+        s = Series([1, 2, 3], index=pd.CategoricalIndex(["A", "B", "C"]))
+        assert s.at['A'] == 1
+        df = DataFrame([[1, 2], [3, 4], [5, 6]],
+                       index=pd.CategoricalIndex(["A", "B", "C"]))
+        assert df.at['B', 1] == 4