TYP: get_indexer (pandas-dev#40612)

* TYP: get_indexer

* update per discussion in pandas-dev#40612

* one more overload

* pre-commit fixup

pandas/core/indexes/base.py

@@ -13,6 +13,7 @@
     Sequence,
     TypeVar,
     cast,
+    overload,
 )
 import warnings
 
@@ -159,6 +160,8 @@
 )
 
 if TYPE_CHECKING:
+    from typing import Literal
+
     from pandas import (
         CategoricalIndex,
         DataFrame,
@@ -5212,7 +5215,8 @@ def set_value(self, arr, key, value):
         """
 
     @Appender(_index_shared_docs["get_indexer_non_unique"] % _index_doc_kwargs)
-    def get_indexer_non_unique(self, target):
+    def get_indexer_non_unique(self, target) -> tuple[np.ndarray, np.ndarray]:
+        # both returned ndarrays are np.intp
         target = ensure_index(target)
 
         if not self._should_compare(target) and not is_interval_dtype(self.dtype):
@@ -5236,7 +5240,7 @@ def get_indexer_non_unique(self, target):
         tgt_values = target._get_engine_target()
 
         indexer, missing = self._engine.get_indexer_non_unique(tgt_values)
-        return ensure_platform_int(indexer), missing
+        return ensure_platform_int(indexer), ensure_platform_int(missing)
 
     @final
     def get_indexer_for(self, target, **kwargs) -> np.ndarray:
@@ -5256,8 +5260,31 @@ def get_indexer_for(self, target, **kwargs) -> np.ndarray:
         indexer, _ = self.get_indexer_non_unique(target)
         return indexer
 
+    @overload
+    def _get_indexer_non_comparable(
+        self, target: Index, method, unique: Literal[True] = ...
+    ) -> np.ndarray:
+        # returned ndarray is np.intp
+        ...
+
+    @overload
+    def _get_indexer_non_comparable(
+        self, target: Index, method, unique: Literal[False]
+    ) -> tuple[np.ndarray, np.ndarray]:
+        # both returned ndarrays are np.intp
+        ...
+
+    @overload
+    def _get_indexer_non_comparable(
+        self, target: Index, method, unique: bool = True
+    ) -> np.ndarray | tuple[np.ndarray, np.ndarray]:
+        # any returned ndarrays are np.intp
+        ...
+
     @final
-    def _get_indexer_non_comparable(self, target: Index, method, unique: bool = True):
+    def _get_indexer_non_comparable(
+        self, target: Index, method, unique: bool = True
+    ) -> np.ndarray | tuple[np.ndarray, np.ndarray]:
         """
         Called from get_indexer or get_indexer_non_unique when the target
         is of a non-comparable dtype.

pandas/core/indexes/category.py

@@ -491,18 +491,23 @@ def _get_indexer(
         limit: int | None = None,
         tolerance=None,
     ) -> np.ndarray:
+        # returned ndarray is np.intp
 
         if self.equals(target):
             return np.arange(len(self), dtype="intp")
 
         return self._get_indexer_non_unique(target._values)[0]
 
     @Appender(_index_shared_docs["get_indexer_non_unique"] % _index_doc_kwargs)
-    def get_indexer_non_unique(self, target):
+    def get_indexer_non_unique(self, target) -> tuple[np.ndarray, np.ndarray]:
+        # both returned ndarrays are np.intp
         target = ibase.ensure_index(target)
         return self._get_indexer_non_unique(target._values)
 
-    def _get_indexer_non_unique(self, values: ArrayLike):
+    def _get_indexer_non_unique(
+        self, values: ArrayLike
+    ) -> tuple[np.ndarray, np.ndarray]:
+        # both returned ndarrays are np.intp
         """
         get_indexer_non_unique but after unrapping the target Index object.
         """
@@ -521,7 +526,7 @@ def _get_indexer_non_unique(self, values: ArrayLike):
             codes = self.categories.get_indexer(values)
 
         indexer, missing = self._engine.get_indexer_non_unique(codes)
-        return ensure_platform_int(indexer), missing
+        return ensure_platform_int(indexer), ensure_platform_int(missing)
 
     @doc(Index._convert_list_indexer)
     def _convert_list_indexer(self, keyarr):