将json.dump() 和json.load()结合使用就会产生如下报错：

import json
# 如果以前存储了用户名，就加载它
#  否则，就提示用户输入用户名并存储它
filename = 'username.json'
try:
   with open(filename) as f_obj:
       username = json.load(f_obj)
except FileNotFoundError:
   username = input("What is your name?")
   filename = 'username.json'
   with open(filename,'w') as f_obj:
       json.dump(username,f_obj)
       print("We'll remember you when you come back, " + username + "!")
else:
   print("Welcome back, " + username + "!")