BUG: Change the location of checking a single-level selection from MultiIndex

Author: Licht-T

pandas/core/groupby.py

@@ -2613,13 +2613,6 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
 
             level = None
             key = group_axis
-        elif key is None:
-            if is_list_like(level) and len(level) == 1:
-                level = level[0]
-
-            if is_scalar(level):
-                key = group_axis.get_level_values(level)
-                level = None
 
     # a passed-in Grouper, directly convert
     if isinstance(key, Grouper):
@@ -2633,6 +2626,14 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     elif isinstance(key, BaseGrouper):
         return key, [], obj
 
+    if key is None and isinstance(group_axis, MultiIndex):
+        if is_list_like(level) and len(level) == 1:
+            level = level[0]
+
+        if level is not None and is_scalar(level):
+            key = group_axis.get_level_values(level)
+            level = None
+
     if not isinstance(key, (tuple, list)):
         keys = [key]
         match_axis_length = False