python - 如何打开 Python pickle 文件、追加、保存和关闭它？

Question

这是我的代码:

 import pickle


 class User:
     def __init__(self, username, password):
         self.username = username
         self.password = password

     def set_password(self):
         self.password = input("Enter NEW password > ")

     def __get_password(self):
         return self.password

     def __get_username(self):
         return self.username

     def change_password(self):
         my_password = input("Enter your CURRENT password > ")
         if my_password == User.__get_password(self):
             self.set_password()
         else:
             print("Please try again")

     def display_details(self):
         print()
         print("Username and password")
         print("---------------------")
         print("username is: ", User.__get_username(self))
         print("password is: ", User.__get_password(self))
         print()

     def __repr__(self):
         return f'username: {self.username}'


 users = [User("MichaelPalin", "P4rr0t"), User("EricIdle", "M0nty"), User("TerryJones", "Pyth0n")]

 try:
     foo = pickle.load(open("users.pickle", "rb"))
 except (OSError, IOError) as f:
     foo = 3
     pickle.dump(foo, open("users.pickle", "wb"))

 # with open('users.pickle', 'wb') as f:
 #     pickle.dump(users, f)


 def find_user(name):
     for user in users:
         if user.username == name:
             return user


 def add_user():
     user = input("Enter NEW user > ")
     password = input(f"Enter password for {user} > ")
     users.append(User(user, password))


 def delete_user():
     delete_user = input("Enter the user you wish to remove > ")
     user = find_user(delete_user)
     if user:
         users.remove(user)
         print('done')
     else:
         print(f'user {delete_user} not found')


 def change_password():
     my_password = input("Enter your CURRENT password > ")
     change_password()


 def display_users():
     for user in users:
         print(user)


 def invalid_entry():  # Response for invalid entries to menu.
     print("Invalid entry, please try again")
     print()


 def menu():  # Display menu, prompt for and accept keyboard choice
     print("Please select one of the following:")
     print()
     print("Enter a if you want to add a new user")
     print("Enter d if you want to delete a user")
     print("Enter f if you want to find a user")
     print("Enter c if you want to change your password")
     print("Enter u if you want to display a list of users")
     print("Enter q if you want to Quit")
     choice = input("")
     return choice


 while True:
     menu_choice = menu()
     if menu_choice.lower() == "a":
         add_user()
     elif menu_choice.lower() == "d":
         delete_user()
     elif menu_choice.lower() == "f":
         find_user()
     elif menu_choice.lower() == "c":
         change_password()
     elif menu_choice.lower() == 'u':
         display_users()
     elif menu_choice.lower() == "q":
         print("Goodbye")
         with open('users.pickle', 'wb') as f:
             pickle.dump(users, f)
             quit()
     else:
         invalid_entry()

但它无法正常工作，因为每次程序运行时 pickle 文件都会被覆盖。我猜罪魁祸首是 users 行，但如果是这样的话，我不知道如何解决问题。我尝试将整行移动到 try 部分的底部，与 pickle.dump 行对齐，但这只是引发了一个错误:

NameError: name 'users' is not defined

有人可以帮我吗？我已经尝试这样做大约两周了，但我越来越沮丧。我在网上查看了很多相关教程，以至于我看到的许多链接现在都是紫色的，但我似乎无法弄清楚如何编写代码来创建正确的逻辑流程。

我认为逻辑可能应该是:

1) 文件是否存在？
如果是 - 什么都不做
如果 N - 使用示例数据创建它

2) 用户是否要创建新的登录名？
如果是 - 将登录名附加到文件
如果 N - 不执行任何操作

Answer 1

我发现的问题是，您实际上并没有在这些行中使用加载的 pickle 数据:

users = [User("MichaelPalin", "P4rr0t"), User("EricIdle", "M0nty"), User("TerryJones", "Pyth0n")]

 try:
     foo = pickle.load(open("users.pickle", "rb"))
 except (OSError, IOError) as f:
     foo = 3
     pickle.dump(foo, open("users.pickle", "wb"))

您将文件加载到 foo 中，但从未使用它，并且您的异常没有多大意义，请尝试将该 block 更改为以下内容:

 try:
     users = pickle.load(open("users.pickle", "rb"))
 except (OSError, IOError) as f:
     users = [User("MichaelPalin", "P4rr0t"), User("EricIdle", "M0nty"), User("TerryJones", "Pyth0n")]
     pickle.dump(foo, open("users.pickle", "wb"))

现在您将尝试加载预制文件，并在失败时生成默认文件