Python登录程序输入非列表密码触发ValueError问题求助

我编写了一个Python登录注册程序，将用户名和密码分别存储在列表中，登录时需要输入用户名和密码进行验证：

import sys
global x
global y

lolusrnm = []
lolpasswrd = []
def login_function():
    while True:
        global usrnm
        usrnm = str(input("please enter your username : "))
        if usrnm in lolusrnm:
            while True:
                global passwrd
                global chkusrnmindx
                global chkpasswrdindx
                passwrd = str(input("Please enter your password : "))
                chkusrnmindx = lolusrnm.index(usrnm)
                chkpasswrdindx = lolpasswrd.index(passwrd)
                if passwrd in lolpasswrd and chkpasswrdindx == chkusrnmindx:
                    print("uou are in ")
                    sys.exit()
                
                elif passwrd not in lolpasswrd:
                    print("password dont match try again")
                else:
                    global quot2
                    print("Wrong password, try again")
                    quot2 = str(input("do u want to continue "))
                    if quot2 == "Y":
                        continue
                    elif quot2 == "N":
                        
                        sys.exit()                  
        else:
            print("Username dosent match..")
def signup_function():
    while True:
        global x
        global y
        print("Enter your username")
        x = str(input())
        lolusrnm.append(x)
        y = str(input("Enter your password : " ))
        lolpasswrd.append(y)
        global sassy
        sassy = input("Do you want to (L)ogin? or continue to (S)ign up another account?")
        if sassy == "L":
            login_function()
        elif sassy == "S":
            continue
        
signup_function()

运行程序完成注册后，使用已注册的用户名登录，输入不在密码列表中的密码时，程序触发以下ValueError错误：

Traceback (most recent call last):
  File "c:\Users\chand\Desktop\PythonBrocode.py", line 52, in <module>       
    signup_function()
  File "c:\Users\chand\Desktop\PythonBrocode.py", line 48, in signup_function
    login_function()
  File "c:\Users\chand\Desktop\PythonBrocode.py", line 18, in login_function 
    chkpasswrdindx = lolpasswrd.index(passwrd)
ValueError: '(whatever password i enter which is not in the list)' is not in list

尽管我已经编写了针对密码不在列表中的处理逻辑：

elif passwrd not in lolpasswrd:
    print("password dont match try again")

我是Python新手，请求帮助排查该问题，谢谢！

问题原因 #

你写的代码里，先执行了chkpasswrdindx = lolpasswrd.index(passwrd)，之后才判断passwrd not in lolpasswrd。当密码不在密码列表中时，list.index()方法会直接抛出ValueError，程序根本走不到后面的elif分支，所以你的错误处理逻辑根本没机会执行。

解决方案 #

方案1：调整代码执行顺序，先判断密码是否存在

把密码存在性的判断放在调用index()方法之前，这样就能避免异常：

import sys

lolusrnm = []
lolpasswrd = []

def login_function():
    while True:
        usrnm = str(input("please enter your username : "))
        if usrnm in lolusrnm:
            while True:
                passwrd = str(input("Please enter your password : "))
                # 先判断密码是否在列表中
                if passwrd not in lolpasswrd:
                    print("password dont match try again")
                    continue
                # 密码存在时再获取索引
                chkusrnmindx = lolusrnm.index(usrnm)
                chkpasswrdindx = lolpasswrd.index(passwrd)
                if chkpasswrdindx == chkusrnmindx:
                    print("you are in ")
                    sys.exit()
                else:
                    print("Wrong password, try again")
                    quot2 = str(input("do u want to continue "))
                    if quot2 == "N":
                        sys.exit()
        else:
            print("Username dosent match..")

def signup_function():
    while True:
        print("Enter your username")
        x = str(input())
        lolusrnm.append(x)
        y = str(input("Enter your password : " ))
        lolpasswrd.append(y)
        sassy = input("Do you want to (L)ogin? or continue to (S)ign up another account?")
        if sassy == "L":
            login_function()
        elif sassy == "S":
            continue

signup_function()

方案2：使用try-except捕获异常

如果不想调整顺序，可以用异常捕获来处理index()方法抛出的错误：

import sys

lolusrnm = []
lolpasswrd = []

def login_function():
    while True:
        usrnm = str(input("please enter your username : "))
        if usrnm in lolusrnm:
            while True:
                passwrd = str(input("Please enter your password : "))
                chkusrnmindx = lolusrnm.index(usrnm)
                try:
                    chkpasswrdindx = lolpasswrd.index(passwrd)
                except ValueError:
                    print("password dont match try again")
                    continue
                if chkpasswrdindx == chkusrnmindx:
                    print("you are in ")
                    sys.exit()
                else:
                    print("Wrong password, try again")
                    quot2 = str(input("do u want to continue "))
                    if quot2 == "N":
                        sys.exit()
        else:
            print("Username dosent match..")

def signup_function():
    while True:
        print("Enter your username")
        x = str(input())
        lolusrnm.append(x)
        y = str(input("Enter your password : " ))
        lolpasswrd.append(y)
        sassy = input("Do you want to (L)ogin? or continue to (S)ign up another account?")
        if sassy == "L":
            login_function()
        elif sassy == "S":
            continue

signup_function()

方案3：改用字典存储（更推荐）

当前用两个列表存储用户名和密码的方式有隐患：如果两个用户的密码相同，index()会返回第一个匹配的索引，导致账号匹配错误。更合理的方式是用字典，直接将用户名和密码作为键值对存储：

import sys

# 用字典存储用户名和密码，键是用户名，值是密码
user_credentials = {}

def login_function():
    while True:
        usrnm = str(input("please enter your username : "))
        if usrnm in user_credentials:
            while True:
                passwrd = str(input("Please enter your password : "))
                if user_credentials[usrnm] == passwrd:
                    print("you are in ")
                    sys.exit()
                else:
                    print("Wrong password, try again")
                    quot2 = str(input("do u want to continue "))
                    if quot2 == "N":
                        sys.exit()
        else:
            print("Username dosent match..")

def signup_function():
    while True:
        print("Enter your username")
        username = str(input())
        if username in user_credentials:
            print("Username already exists, choose another one")
            continue
        password = str(input("Enter your password : " ))
        user_credentials[username] = password
        sassy = input("Do you want to (L)ogin? or continue to (S)ign up another account?")
        if sassy == "L":
            login_function()
        elif sassy == "S":
            continue

signup_function()

这个版本不仅避免了索引匹配的问题，代码逻辑也更简洁。

内容的提问来源于stack exchange，提问作者chnkrn287