Update app.py

app.py

@@ -29,7 +29,7 @@ def init(filename=filename,save_data=save_data):
     return lod
 
 
-def sort_doc(in_list: list, steps_in: int, control: int, prev_list: str):
+def sort_doc(in_list: list, steps_in: int, control: int, prev_list: str=None):
     prev_list=init()
     control_json={'control':'0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ','char':'','leng':62}
     key_cnt=len(in_list)