REF: Use _maybe_cast_indexer and dont override Categorical.get_loc (#31681)

Author: jbrockmendel

pandas/core/indexes/category.py

@@ -439,44 +439,10 @@ def _to_safe_for_reshape(self):
         """ convert to object if we are a categorical """
         return self.astype("object")
 
-    def get_loc(self, key, method=None):
-        """
-        Get integer location, slice or boolean mask for requested label.
-
-        Parameters
-        ----------
-        key : label
-        method : {None}
-            * default: exact matches only.
-
-        Returns
-        -------
-        loc : int if unique index, slice if monotonic index, else mask
-
-        Raises
-        ------
-        KeyError : if the key is not in the index
-
-        Examples
-        --------
-        >>> unique_index = pd.CategoricalIndex(list('abc'))
-        >>> unique_index.get_loc('b')
-        1
-
-        >>> monotonic_index = pd.CategoricalIndex(list('abbc'))
-        >>> monotonic_index.get_loc('b')
-        slice(1, 3, None)
-
-        >>> non_monotonic_index = pd.CategoricalIndex(list('abcb'))
-        >>> non_monotonic_index.get_loc('b')
-        array([False,  True, False,  True], dtype=bool)
-        """
+    def _maybe_cast_indexer(self, key):
         code = self.categories.get_loc(key)
         code = self.codes.dtype.type(code)
-        try:
-            return self._engine.get_loc(code)
-        except KeyError:
-            raise KeyError(key)
+        return code
 
     def get_value(self, series: "Series", key: Any):
         """

pandas/tests/indexes/categorical/test_indexing.py

@@ -172,6 +172,23 @@ def test_get_loc(self):
             with pytest.raises(KeyError, match="'c'"):
                 i.get_loc("c")
 
+    def test_get_loc_unique(self):
+        cidx = pd.CategoricalIndex(list("abc"))
+        result = cidx.get_loc("b")
+        assert result == 1
+
+    def test_get_loc_monotonic_nonunique(self):
+        cidx = pd.CategoricalIndex(list("abbc"))
+        result = cidx.get_loc("b")
+        expected = slice(1, 3, None)
+        assert result == expected
+
+    def test_get_loc_nonmonotonic_nonunique(self):
+        cidx = pd.CategoricalIndex(list("abcb"))
+        result = cidx.get_loc("b")
+        expected = np.array([False, True, False, True], dtype=bool)
+        tm.assert_numpy_array_equal(result, expected)
+
 
 class TestGetIndexer:
     def test_get_indexer_base(self):