with open('abc.txt') as f:
    credentials = [x.strip().split(':') for x in f.readlines()]

for username, password in credentials:

    user_input = input('Please Enter username: ')

    if user_input != username:

        sys.exit('Incorrect incorrect username, terminating... n')


    user_input = input('Please Enter Password: ')



    if user_input != password:

        sys.exit('Incorrect Password, terminating... n')



    print ('User is logged in!n')

Sil:xyz123
smith:abc321

with open('abc.txt') as f:
    credentials = dict([x.strip().split(':') for x in f.readlines()]) # Created a dictionary with username:password items

username_input = input('Please Enter username: ')

if username_input not in credentials:  # Check if username is in the credentials dictionary

        sys.exit('Incorrect incorrect username, terminating... n')

password_input = input('Please Enter Password: ')

if password_input != credentials[username_input]: # Check if the password entered matches the password in the dictionary

        sys.exit('Incorrect Password, terminating... n')

print ('User is logged in!n')