By convention, tuples indicate multi-level lookups. This change fixes nlargest/smallest functionality for dataframes with MultiIndex columns.

Author: Kristian Holsheimer

pandas/core/algorithms.py

@@ -1178,7 +1178,7 @@ class SelectNFrame(SelectN):
 
     def __init__(self, obj, n, keep, columns):
         super(SelectNFrame, self).__init__(obj, n, keep)
-        if not is_list_like(columns):
+        if not is_list_like(columns) or isinstance(columns, tuple):
             columns = [columns]
         columns = list(columns)
         self.columns = columns