REF: implement MultiIndex._get_indexer_level_0 (#42480)

Author: jbrockmendel

pandas/core/indexes/multi.py

@@ -73,7 +73,6 @@
 from pandas.core.arrays import Categorical
 from pandas.core.arrays.categorical import factorize_from_iterables
 import pandas.core.common as com
-from pandas.core.indexers import is_empty_indexer
 import pandas.core.indexes.base as ibase
 from pandas.core.indexes.base import (
     Index,
@@ -2559,24 +2558,29 @@ def _convert_listlike_indexer(self, keyarr) -> np.ndarray | None:
 
         # are we indexing a specific level
         if len(keyarr) and not isinstance(keyarr[0], tuple):
-            _, indexer = self.reindex(keyarr, level=0)
-
-            # take all
-            if indexer is None:
-                indexer = np.arange(len(self), dtype=np.intp)
-                return indexer
-
-            check = self.levels[0].get_indexer(keyarr)
-            mask = check == -1
+            indexer = self._get_indexer_level_0(keyarr)
+            mask = indexer == -1
             if mask.any():
-                raise KeyError(f"{keyarr[mask]} not in index")
-            elif is_empty_indexer(indexer, keyarr):
+                check = self.levels[0].get_indexer(keyarr)
+                cmask = check == -1
+                if cmask.any():
+                    raise KeyError(f"{keyarr[cmask]} not in index")
                 # We get here when levels still contain values which are not
                 # actually in Index anymore
                 raise KeyError(f"{keyarr} not in index")
 
         return indexer
 
+    def _get_indexer_level_0(self, target) -> np.ndarray:
+        """
+        Optimized equivalent to `self.get_level_values(0).get_indexer_for(target)`.
+        """
+        lev = self.levels[0]
+        codes = self._codes[0]
+        cat = Categorical.from_codes(codes=codes, categories=lev)
+        ci = Index(cat)
+        return ci.get_indexer_for(target)
+
     def _get_partial_string_timestamp_match_key(self, key):
         """
         Translate any partial string timestamp matches in key, returning the