python10.4.3

10.4.3重构
错误实例

import jsondef laoyonghu():      #获取老用户名字filename='username.json'with open(filename) as f:username=json.load(f)return usernamedef xinyonghu():   #获取新用户名字filename='username.json'username=input("what's your name:")with open(filename,'w') as f:json.dump(username,f)return usernamedef dazhaohu(): #给新或老用户打招呼username=laoyonghu()if username:       #如果有老用户（为真）print(f"welcome {username}.")else:username=xinyonghu()print(f"we'll remember you when you come back, {username}.")dazhaohu()

以上代码的运行顺序，是运行打招呼函数时，先调用老用户函数，再调用新用户函数。如果调用老用户函数时，filename.json文件不存在，则会导致FileNotFoundError程序出错，无法进入else新用户print部分。

为避免出现这种情况，需要在获取老用户的函数中，使用try-except代码块，防止因文件不存在导致错误，表达为except FileNotFoundError后return None，else之后return username。否则运行打招呼函数时，会因为老用户函数中的filename不存在而直接导致FileNotFoundError，

修改后：

import jsondef laoyonghu():filename='username.json'try:with open(filename) as f:username=json.load(f)except FileNotFoundError:return Noneelse:return usernamedef xinyonghu():filename='username.json'username=input("what's your name:")with open(filename,'w') as f:json.dump(username,f)return usernamedef dazhaohu():username=laoyonghu()if username:print(f"welcome {username}.")else:username=xinyonghu()print(f"we'll remember you when you come back, {username}.")dazhaohu()