使用Python编写一个程序来验证用户名和密码

以下是一个使用Python编写的用于验证用户名和密码的程序，包括了一个使用例子。

import getpass

def validate_username(username):
    if len(username) < 5 or len(username) > 15:
        return False
    return True

def validate_password(password):
    if len(password) < 8 or len(password) > 20:
        return False
    if not any(char.isdigit() for char in password):
        return False
    if not any(char.isalpha() for char in password):
        return False
    return True

def signup():
    print("Sign up:")
    username = input("Enter username: ")
    while not validate_username(username):
        print("Invalid username! Username should be 5-15 characters long.")
        username = input("Enter username: ")
    
    password = getpass.getpass("Enter password: ")
    while not validate_password(password):
        print("Invalid password! Password should be 8-20 characters long and contain at least one letter and one digit.")
        password = getpass.getpass("Enter password: ")
    
    print("Successfully signed up!")

def login():
    print("Login:")
    username = input("Enter username: ")
    password = getpass.getpass("Enter password: ")
    
    if username == "admin" and password == "admin123":
        print("Welcome, admin!")
    else:
        print("Invalid username or password!")

def main():
    print("Welcome to User Login System!")
    choice = input("Select an option (1 - Sign up, 2 - Login): ")
    
    if choice == "1":
        signup()
    elif choice == "2":
        login()
    else:
        print("Invalid choice! Please try again.")

if __name__ == "__main__":
    main()

使用例子：

Welcome to User Login System!
Select an option (1 - Sign up, 2 - Login): 1
Sign up:
Enter username: john123
Enter password: 
Invalid password! Password should be 8-20 characters long and contain at least one letter and one digit.
Enter password: password123
Successfully signed up!

Welcome to User Login System!
Select an option (1 - Sign up, 2 - Login): 2
Login:
Enter username: john123
Enter password: password123
Invalid username or password!

Welcome to User Login System!
Select an option (1 - Sign up, 2 - Login): 2
Login:
Enter username: admin
Enter password: admin123
Welcome, admin!