如何从文本文件中的不同行写入和读取？

Question

I have a very simple 'login' program that I've almost got finished. I'm trying to get my make_acc() function to write the username on line 1 and the password on line 2, as well as make my login() function read those separate lines per what needs to be checked. I'm pretty sure the answer has to do with marking the readline command for which line needs to be read, but I'm not sure how to implement it in my code properly. Here's the code.

# This function has the user input a username for their account
def make_acc():
    username = input('Make a username:')
    file = open('acc_data.txt','w')
    file.write(username)
    file.close()
    #password = input('Make a password:')
    #file = open('acc_data.txt','w')
    #file.write(password)

# This function has the user login to a preexisting account
def login():
    input_user = input('Enter your username:')
    file = open('acc_data.txt','r')
    username = file.readline()
    if input_user == username:
        print('You are now logged in')
    else:
        print('That user does not exist')
        login()

# This variable will be defined as a yes or no depending on whether or not the use has an account
acc_bool = input('Do you already have an account?:')

# This if statement runs the login() function if the user answered yes to the previous input
if acc_bool == 'yes':
    login()
# This elif statement runs the make_acc() function if the user answered no to the previous input
elif acc_bool == 'no':
    make_acc()
    login()

Answer 1

This should do it:

def make_acc():
    username = input('Make a username:')
    password = input('Make a password:')
    with open('acc_data.txt','a') as file:
        file.write(username+'\n')
        file.write(password)

def login():
    input_user = input('Enter your username:')
    with open('acc_data.txt','r') as file:
        if input_user in [u for i,u in enumerate(file.readlines()) if not u%2]:    
            print('You are now logged in')
        else:
            print('That user does not exist')
            login()

Instead of having the usernames and passwords all in 2 lines,
you can use indexes to determine whether a line is a username or password:
even indexes are for usernames, and odd ones are for passwords.

---

UPDATE:

This part: [u for i,u in enumerate(file.readlines()) if not u%2] lists all the strings in file.readlines() (a list of all the lines in f.read()) if the index of the string, i , doesn't leave a remainder when divided by 2.

You see, enumerate() will basically let us iterate through an array and let use easily access the index of the current iteration.

Answer 2

First, uncomment the part of make_acc that asks for a password and add a file.close() .

Then, notice that when you've run the program and inputted the two pieces of information, only one is left in the file, this is because, when calling open() , you use the w mode, which truncates the file before allowing you to write. The consequence of this is that when you open the file again a couple of lines later, all information previously stored is lost. The solution is to use mode r+ or a or just not close the file until the end of the function, thereby avoiding having to reopening it when writing the password.

Next, when reading from the file, you can use file.readline() just as you already are doing. Choosing what line to read happens by default, because every call to readline advances what line is being read. This means that the second time you call it since you opened the file, the second line will be read.