REF: de-duplicate MultiIndex.reindex (pandas-dev#42226)

Author: jbrockmendel

pandas/core/indexes/base.py

@@ -3781,14 +3781,25 @@ def reindex(
         target = ensure_has_len(target)  # target may be an iterator
 
         if not isinstance(target, Index) and len(target) == 0:
-            target = self[:0]
+            if level is not None and self._is_multi:
+                # "Index" has no attribute "levels"; maybe "nlevels"?
+                idx = self.levels[level]  # type: ignore[attr-defined]
+            else:
+                idx = self
+            target = idx[:0]
         else:
             target = ensure_index(target)
 
         if level is not None:
             if method is not None:
                 raise TypeError("Fill method not supported if level passed")
-            _, indexer, _ = self._join_level(target, level, how="right")
+
+            # TODO: tests where passing `keep_order=not self._is_multi`
+            #  makes a difference for non-MultiIndex case
+            target, indexer, _ = self._join_level(
+                target, level, how="right", keep_order=not self._is_multi
+            )
+
         else:
             if self.equals(target):
                 indexer = None
@@ -3797,6 +3808,8 @@ def reindex(
                     indexer = self.get_indexer(
                         target, method=method, limit=limit, tolerance=tolerance
                     )
+                elif self._is_multi:
+                    raise ValueError("cannot handle a non-unique multi-index!")
                 else:
                     if method is not None or limit is not None:
                         raise ValueError(
@@ -3805,11 +3818,18 @@ def reindex(
                         )
                     indexer, _ = self.get_indexer_non_unique(target)
 
+        target = self._wrap_reindex_result(target, indexer, preserve_names)
+        return target, indexer
+
+    def _wrap_reindex_result(self, target, indexer, preserve_names: bool):
+        target = self._maybe_preserve_names(target, preserve_names)
+        return target
+
+    def _maybe_preserve_names(self, target: Index, preserve_names: bool):
         if preserve_names and target.nlevels == 1 and target.name != self.name:
-            target = target.copy()
+            target = target.copy(deep=False)
             target.name = self.name
-
-        return target, indexer
+        return target
 
     @final
     def _reindex_non_unique(

pandas/core/indexes/multi.py

@@ -2476,51 +2476,7 @@ def sortlevel(
 
         return new_index, indexer
 
-    def reindex(
-        self, target, method=None, level=None, limit=None, tolerance=None
-    ) -> tuple[MultiIndex, np.ndarray | None]:
-        """
-        Create index with target's values (move/add/delete values as necessary)
-
-        Returns
-        -------
-        new_index : pd.MultiIndex
-            Resulting index
-        indexer : np.ndarray[np.intp] or None
-            Indices of output values in original index.
-
-        """
-        # GH6552: preserve names when reindexing to non-named target
-        # (i.e. neither Index nor Series).
-        preserve_names = not hasattr(target, "names")
-
-        if level is not None:
-            if method is not None:
-                raise TypeError("Fill method not supported if level passed")
-
-            # GH7774: preserve dtype/tz if target is empty and not an Index.
-            # target may be an iterator
-            target = ibase.ensure_has_len(target)
-            if len(target) == 0 and not isinstance(target, Index):
-                idx = self.levels[level]
-                target = idx[:0]
-            else:
-                target = ensure_index(target)
-            target, indexer, _ = self._join_level(
-                target, level, how="right", keep_order=False
-            )
-        else:
-            target = ensure_index(target)
-            if self.equals(target):
-                indexer = None
-            else:
-                if self.is_unique:
-                    indexer = self.get_indexer(
-                        target, method=method, limit=limit, tolerance=tolerance
-                    )
-                else:
-                    raise ValueError("cannot handle a non-unique multi-index!")
-
+    def _wrap_reindex_result(self, target, indexer, preserve_names: bool):
         if not isinstance(target, MultiIndex):
             if indexer is None:
                 target = self
@@ -2531,16 +2487,20 @@ def reindex(
                     target = MultiIndex.from_tuples(target)
                 except TypeError:
                     # not all tuples, see test_constructor_dict_multiindex_reindex_flat
-                    return target, indexer
+                    return target
+
+        target = self._maybe_preserve_names(target, preserve_names)
+        return target
+
+    def _maybe_preserve_names(self, target: Index, preserve_names: bool):
         if (
             preserve_names
             and target.nlevels == self.nlevels
             and target.names != self.names
         ):
             target = target.copy(deep=False)
             target.names = self.names
-
-        return target, indexer
+        return target
 
     # --------------------------------------------------------------------
     # Indexing Methods