REF: de-duplicate MultiIndex validation (#39425)

Author: jbrockmendel

pandas/core/indexes/multi.py

@@ -881,16 +881,7 @@ def set_levels(self, levels, level=None, inplace=None, verify_integrity=True):
         if is_list_like(levels) and not isinstance(levels, Index):
             levels = list(levels)
 
-        if level is not None and not is_list_like(level):
-            if not is_list_like(levels):
-                raise TypeError("Levels must be list-like")
-            if is_list_like(levels[0]):
-                raise TypeError("Levels must be list-like")
-            level = [level]
-            levels = [levels]
-        elif level is None or is_list_like(level):
-            if not is_list_like(levels) or not is_list_like(levels[0]):
-                raise TypeError("Levels must be list of lists-like")
+        level, levels = _require_listlike(level, levels, "Levels")
 
         if inplace:
             idx = self
@@ -1050,16 +1041,7 @@ def set_codes(self, codes, level=None, inplace=None, verify_integrity=True):
         else:
             inplace = False
 
-        if level is not None and not is_list_like(level):
-            if not is_list_like(codes):
-                raise TypeError("Codes must be list-like")
-            if is_list_like(codes[0]):
-                raise TypeError("Codes must be list-like")
-            level = [level]
-            codes = [codes]
-        elif level is None or is_list_like(level):
-            if not is_list_like(codes) or not is_list_like(codes[0]):
-                raise TypeError("Codes must be list of lists-like")
+        level, codes = _require_listlike(level, codes, "Codes")
 
         if inplace:
             idx = self
@@ -3870,3 +3852,20 @@ def _coerce_indexer_frozen(array_like, categories, copy: bool = False) -> np.nda
         array_like = array_like.copy()
     array_like.flags.writeable = False
     return array_like
+
+
+def _require_listlike(level, arr, arrname: str):
+    """
+    Ensure that level is either None or listlike, and arr is list-of-listlike.
+    """
+    if level is not None and not is_list_like(level):
+        if not is_list_like(arr):
+            raise TypeError(f"{arrname} must be list-like")
+        if is_list_like(arr[0]):
+            raise TypeError(f"{arrname} must be list-like")
+        level = [level]
+        arr = [arr]
+    elif level is None or is_list_like(level):
+        if not is_list_like(arr) or not is_list_like(arr[0]):
+            raise TypeError(f"{arrname} must be list of lists-like")
+    return level, arr