Proof of performance

Author: tp

pandas/core/indexes/base.py

@@ -2944,6 +2944,10 @@ def _get_unique_index(self, dropna=False):
         -------
         loc : int if unique index, slice if monotonic index, else mask
 
+        Raises
+        ------
+        KeyError : If key is not in self
+
         Examples
         ---------
         >>> unique_index = pd.Index(list('abc'))

pandas/core/indexes/category.py

@@ -415,6 +415,10 @@ def get_loc(self, key, method=None):
         -------
         loc : int if unique index, slice if monotonic index, else mask
 
+        Raises
+        ------
+        KeyError : If key is not in self
+
         Examples
         ---------
         >>> unique_index = pd.CategoricalIndex(list('abc'))
@@ -432,6 +436,14 @@ def get_loc(self, key, method=None):
         codes = self.categories.get_loc(key)
         if (codes == -1):
             raise KeyError(key)
+
+        if self.is_monotonic_increasing and not self.is_unique:
+            if codes not in self._engine:
+                raise KeyError(key)
+            codes = self.codes.dtype.type(codes)
+            lhs = self.codes.searchsorted(codes, side='left')
+            rhs = self.codes.searchsorted(codes, side='right')
+            return slice(lhs, rhs)
         return self._engine.get_loc(codes)
 
     def get_value(self, series, key):