break 不会中断 while 循环。 为什么？

Question

I'm trying to do a login system from text file.
When I enter wrong input that isn't in the text it does what I want it to do, which is to output "wrong username and password" and "try again" and ask for username and pass again.
But when I enter correct details, the loop never breaks. It keeps asking for user and pass.
Here is my code:

def check():
        users = open('student.txt').read().split('\n')
        for i in range(len(users)): users[i] = users[i].split(',')


        while True:
            username = str(input('Username: '))
            password = str(input('Password: '))

            for user in users:
                uname = user[3]
                pword = user[4]

                if uname == username and pword == password:
                    print('Hello ' + user[0] + '.')
                    print('You are logged in as: ' + user[3] +  '.')
                    break
                
                    

            else:
                print('Wrong username/password.')
                print('Try again!\n\n')
    check()

Answer 1

The break always leaves the immediate loop construct, the "innermost".
For your code that is the for loop, not the while loop which contains it.

Answer 2

When using a break statement, it will only exit the inner loop it is currently in, which means it will still be looping the outer loop. The common approach for this problem would be to refractor your nested loop into a function and use a return statement to exit the code.

Another approach would be adding some continue statement to avoid reaching a second break and only reach it when the inner loop is exited. Something like this:

def check():
      users = open('student.txt').read().split('\n')
       for i in range(len(users)):
            users[i] = users[i].split(',')

        while True:
            username = str(input('Username: '))
            password = str(input('Password: '))

            for user in users:
                uname = user[3]
                pword = user[4]

                if uname == username and pword == password:
                    print('Hello ' + user[0] + '.')
                    print('You are logged in as: ' + user[3] + '.')
                    break
            else:
                print('Wrong username/password.')
                print('Try again!\n\n')
                
                # Continue if the inner loop wasn't broken.
                continue

            # Inner loop was broken, break the outer.
            break

Answer 3

the break statement will break out of while AND for loops. As frustrating as it might seem, these break statements will only break out of the closest loop. If you take a look at your code, you can see that your break statement is actually called inside of a for loop within your while loop . This means you are breaking out of the for loop but the while loop will continue on as normal.

One great alternative to deal with this issue is to turn your while loop into a function and replace your break statement with a return statement. This way your while loop will also stop!

Answer 4

As said in my comment, your break statement is within a for loop that's within a while loop. break statements won't break any outer loops.

Here is a possible solution

def check():
        users = open('student.txt').read().split('\n')
        for i in range(len(users)): users[i] = users[i].split(',')

        username = str(input('Username: '))
        password = str(input('Password: '))

        for user in users:
            uname = user[3]
            pword = user[4]

             while True:

                if uname == username and pword == password:
                    print('Hello ' + user[0] + '.')
                    print('You are logged in as: ' + user[3] +  '.')
                    break
            
                else:
                    print('Wrong username/password.')
                    print('Try again!\n\n')

check()