import json

def get_stored_username():

"""如果存储了用户名，就获取它"""

filename = 'username.json'

try:

with open(filename) as f_obj:

username = json.load(f_obj)

except FileNotFoundError:

return None

else:

return username

def get_new_username():

"""提示用户输入用户名"""

username = input("What is your name? ")

filename = 'username.json'

with open(filename, 'w') as f_obj:

json.dump(username, f_obj)

return username

def greet_user():

"""问候用户，并指出其名字"""

username = get_stored_username()

if username:

print("Welcome back, " + username + "!")

else:

username = get_new_username()

print("We'll remember you when you come back, " + username + "!")

greet_user()