TYP: Index._concat (#37671)

Author: jbrockmendel

pandas/core/indexes/base.py

@@ -4297,13 +4297,13 @@ def append(self, other):
 
         return self._concat(to_concat, name)
 
-    def _concat(self, to_concat, name):
+    def _concat(self, to_concat: List["Index"], name: Label) -> "Index":
         """
         Concatenate multiple Index objects.
         """
-        to_concat = [x._values if isinstance(x, Index) else x for x in to_concat]
+        to_concat_vals = [x._values for x in to_concat]
 
-        result = concat_compat(to_concat)
+        result = concat_compat(to_concat_vals)
         return Index(result, name=name)
 
     def putmask(self, mask, value):

pandas/core/indexes/category.py

@@ -653,7 +653,7 @@ def map(self, mapper):
         mapped = self._values.map(mapper)
         return Index(mapped, name=self.name)
 
-    def _concat(self, to_concat, name):
+    def _concat(self, to_concat: List["Index"], name: Label) -> "CategoricalIndex":
         # if calling index is category, don't check dtype of others
         codes = np.concatenate([self._is_dtype_compat(c).codes for c in to_concat])
         cat = self._data._from_backing_data(codes)