如何防止重复写入.txt文件

Question

Python 新手和第一次发帖者（但长期潜伏）在这里，对于任何格式或协议问题深表歉意。

我正在尝试编写一段代码来防止将重复的用户名写入文件。我已经尝试了论坛上以前类似问题的各种解决方案，但无济于事。

在迄今为止的最新迭代中，我已成功拒绝第一个密钥的重复项，但后续密钥的重复项仍在处理中。即，如果我的密钥具有值“admin”和“user”（我在第一个打印语句中已确认），则 x = admin 正确地被拒绝，但 x = users 仍在进行中。请帮忙

我的代码：

alr_exists = {}
user = {} 
with open('user.txt', 'r') as file: 
    for lines in file:
        (keys, values) = lines.split()
        keys = keys.replace(',', '')
        alr_exists[keys] = keys
        print(alr_exists[keys])

        while True: 
            x = input("To register a new user, please enter a username:\n").lower()
            if x in alr_exists[keys]:
                print(f'Sorry, the user name {x} has already been taken. Please try again.\n')
                continue
            else:
                y = input("Please enter a password:\n").lower()
                z = input("To confirm, please reenter your password:\n").lower()

样本数据集：
管理员，adm1n
用户，us3r
（其中第一列包含用户名和第二列密码）

Answer 1

我希望您喜欢使用 Python 编程。我也是新来的。

查看您的代码，我发现三件事可能导致其无法正常工作：

1. 您有一个无限的 while 循环，并且根据缩进，它也在逐行读取的 for 循环内部。有些事情可以在周期内进行，而另一些事情是不必要的，并且容易犯错误。
2. 您将现有用户名存储在字典中，但它同时具有用户名的键和值，也许您可以使用列表或为每个键分配另一个值，例如 True/False 或密码。
3. 您通过将用户与上次迭代的值进行比较来检查用户是否已经存在，在代码中您没有访问键，而是访问文件最后一行的值（用户名）。

   if x in alr_exists[keys]:

   您可以在您的情况下使用 dict.keys() 函数

   if x in alr_exists.keys()

我提出了你的代码的修改版本，我希望它对你来说并不复杂，它也有注释并且遵循pep8的风格。

def read_existing_usernames(filename):
    """Read existing usernames from a file and store them in a dictionary.

    Args:
        filename (str): The name of the file containing existing usernames.

    Returns:
        dict: A dictionary containing existing usernames as keys.
    """
    # Create an empty dictionary to store existing usernames
    existing_usernames = {}
    
    # Read existing usernames from the file
    with open(filename, 'r') as file:
        # Iterate over each line in the file
        for line in file:
            # Split the line into username and password (separated by a space)
            username, _ = line.split()
            
            # Store the username in the dictionary and set the value to True
            existing_usernames[username] = True
    
    # Print a success message with the existing usernames
    print(f'Existing usernames: {', '.join(existing_usernames)}')
    
    # Return the dictionary of existing usernames
    return existing_usernames


def register_user():
    """
    Register a new user by entering a username and password.
    The username must be unique and the password must be confirmed.
    The new username and password are stored in a file. ('user.txt')
    
    Args:
        None
        
    Returns:
        None
    """
    # Bring in existing usernames from the file with the function
    existing_usernames = read_existing_usernames('user.txt')
    
    # Iterate until a new user is successfully registered
    while True:
        # Prompt the user to enter a new username
        username = input("To register a new user, please enter a username: ")
        
        # Check if the username is already taken
        if username.lower() in existing_usernames:
            # Print an error message and prompt the user to try again
            print(f"Sorry, the username '{username}' has already been taken")
            print("Please try again.\n")
        # If the username is not taken
        else:
            # Prompt the user to enter a password and confirm it
            password = input("Please enter a password: ")
            
            # Confirm the password by reentering it
            confirm_password = input("Please reenter your password: ")
            
            # Check if the passwords match
            if password == confirm_password:
                # Open the file in append mode and write the new information
                # Append because we don't want to overwrite the existing data
                with open('user.txt', 'a') as file:
                    # Write the new username and password to the file
                    file.write(f'{username} {password}\n')
                
                # Print a success register message
                print("User registered successfully!")
                
                # Break out of the loop to end the program
                break
            
            # If the passwords do not match
            else:
                # Print an error message and prompt the user to try again
                print("Passwords do not match. Please try again.\n")

# Run main program to register a new user
register_user()