BUG: don't silently ignore kwargs in get_indexer_for (#42310)

Author: jbrockmendel

doc/source/whatsnew/v1.4.0.rst

@@ -87,7 +87,7 @@ See :ref:`install.dependencies` and :ref:`install.optional_dependencies` for mor
 
 Other API changes
 ^^^^^^^^^^^^^^^^^
--
+- :meth:`Index.get_indexer_for` no longer accepts keyword arguments (other than 'target'); in the past these would be silently ignored if the index was not unique (:issue:`42310`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/indexes/base.py

@@ -5317,7 +5317,7 @@ def get_indexer_non_unique(self, target) -> tuple[np.ndarray, np.ndarray]:
         return ensure_platform_int(indexer), ensure_platform_int(missing)
 
     @final
-    def get_indexer_for(self, target, **kwargs) -> np.ndarray:
+    def get_indexer_for(self, target) -> np.ndarray:
         """
         Guaranteed return of an indexer even when non-unique.
 
@@ -5330,7 +5330,7 @@ def get_indexer_for(self, target, **kwargs) -> np.ndarray:
             List of indices.
         """
         if self._index_as_unique:
-            return self.get_indexer(target, **kwargs)
+            return self.get_indexer(target)
         indexer, _ = self.get_indexer_non_unique(target)
         return indexer