json文件读写函数

老师代码：

import json
def op_data(filename,dic=None):
    if dic:#写入进去
        with open(filename,'w',encoding='utf-8') as fw:
            json.dump(dic,fw,ensure_ascii=False,indent=4) #indent代码缩进几位
    else:
        with open(filename,encoding='utf-8') as fr:
            return json.load(fr)
FILE_NAME = 'user_info.json'
all_users = op_data(FILE_NAME)
for i in range(3):
    choice = input('输入，1注册，2、删除')
    if choice=='1':
        username = input('usenrame:')
        pwd = input('pwd:')
        if username not in all_users:
            all_users[username]=pwd
            op_data(FILE_NAME,all_users)
    elif choice=="2":
        username = input('usenrame:')
        all_users.pop(username)
        op_data(FILE_NAME, all_users)