我怎样才能 go 回到程序已经执行的行？

Question

I am making a basic sign up and login in system, right now I'm building the log in part, the program asks the user for an input of a username which then checks the database of usernames, if the username is found it would then check for the password(going to work on that) but for now I'm looking at what would happen if the username was not found, okay it would ask again in case there was a spelling error after that it would ask the user for an input to whether they would like to back to the main menu but the issue is the main menu has already been executed at the beginning of the program and if I just copied and pasted it would make the program much longer and would be a paradox. If it's still unclear please let me now.

This is my program:

import sys, re, csv
from re import match
def isUsernameValid(username):
    isValid = match(r"^[A-Za-z0-9_]{3,16}$", username)
    if isValid:
        print("True")
        return True
    else:
        print("False")
        return False
def isUsernameFree(username):
        with open('accountdatabase.txt', mode = 'r') as file:
                reader = csv.reader(file, delimiter=',')
                for line in file:
                        if username == line.split(',')[0]:
                                print("False")
                                return False

                        else:
                                print("True")
                                return True


usernamecheck = False
charcheck = False
menu = int(input("1. Sign Up\n2. Log in\n3. Exit\nInput: "))
menu_numbers = (1,2,3)
while menu not in menu_numbers:
    menu = int(input("1. Sign Up\n2. Log in\n3. Exit\nInput: "))
if menu == 1:
        newusername = input("Input a new username: ")
        usernamecheck = isUsernameValid(newusername) and isUsernameFree(newusername)
        while usernamecheck == False:
                newusername = input("Input a new username: ")
                usernamecheck = isUsernameValid(newusername) and isUsernameFree(newusername)
        newpassword = input("Input a password: ")
        while len(newpassword)<8:
               newpassword = input("Input a password that has 8 or more characters: ")
        validatepassword = input("Input the same password: ")
        while newpassword != validatepassword:
                newpassword = input("Input a password: ")
                while len(newpassword)<8:
                        newpassword = input("Input a password that has 8 or more characters: ")
                validatepassword = input("Input the same password: ")
        with open('accountdatabase.txt', mode = 'a') as file:
                file.write(str(newusername) + "," + str(newpassword))

elif menu == 2:
    usernamesearch = input("Username: ")
    with open('accountdatabase.txt', mode = 'r') as file:
        reader = csv.reader(file, delimiter=',')
        for line in file:
            if usernamesearch == line.split(',')[0]:
                print(f'Username ({usernamesearch}) found.')
                accountfound = True
            else:
                print(f'Username not found.')
                accountfound = False

    while accountfound == False:
        usernamesearch = input("Username: ")
        with open('accountdatabase.txt', mode = 'r') as file:
            reader = csv.reader(file, delimiter=',')
            for line in file:
                if usernamesearch == line.split(',')[0]:
                    print(f'Username ({usernamesearch}) found.')
                    accountfound = True
                else:
                    print(f'Username not found.')
                    accountfound = False
                    menu = input("Choose 1 to go back to menu and 2 to keep trying: ")

As you can see at the end I ask for an input which the user can determine whether they want to start all over again at the beginning where they could sign up instead or try another username but I have no clue how to do that. Thank you.

Answer 1

Just write everything into a while loop.

while (True):
    usernamecheck = False
    charcheck = False
    menu = int(input("1. Sign Up\n2. Log in\n3. Exit\nInput: "))
    menu_numbers = (1,2,3)
    while menu not in menu_numbers:
        menu = int(input("1. Sign Up\n2. Log in\n3. Exit\nInput: "))
    if menu == 1:
            newusername = input("Input a new username: ")
            usernamecheck = isUsernameValid(newusername) and isUsernameFree(newusername)
            while usernamecheck == False:
                    newusername = input("Input a new username: ")
                    usernamecheck = isUsernameValid(newusername) and isUsernameFree(newusername)
            newpassword = input("Input a password: ")
            while len(newpassword)<8:
                newpassword = input("Input a password that has 8 or more characters: ")
            validatepassword = input("Input the same password: ")
            while newpassword != validatepassword:
                    newpassword = input("Input a password: ")
                    while len(newpassword)<8:
                            newpassword = input("Input a password that has 8 or more characters: ")
                    validatepassword = input("Input the same password: ")
            with open('accountdatabase.txt', mode = 'a') as file:
                    file.write(str(newusername) + "," + str(newpassword))

    elif menu == 2:
        usernamesearch = input("Username: ")
        with open('accountdatabase.txt', mode = 'r') as file:
            reader = csv.reader(file, delimiter=',')
            for line in file:
                if usernamesearch == line.split(',')[0]:
                    print(f'Username ({usernamesearch}) found.')
                    accountfound = True
                else:
                    print(f'Username not found.')
                    accountfound = False

        while accountfound == False:
            usernamesearch = input("Username: ")
            with open('accountdatabase.txt', mode = 'r') as file:
                reader = csv.reader(file, delimiter=',')
                for line in file:
                    if usernamesearch == line.split(',')[0]:
                        print(f'Username ({usernamesearch}) found.')
                        accountfound = True
                    else:
                        print(f'Username not found.')
                        accountfound = False
                        menu = input("Choose 1 to go back to menu and 2 to keep trying: ")

When you want to exit the loop just break it. eg

elif menu == 3:
    break

Alternatively you could create a boolean exit = False before the loop, change the exit condition of your loop to while(not exit) and set exit = True when you want to exit it.