Add user input to list and then save to file

#First run this program to set up the files

import pickle

pickle.dump([],open("usernames.dat","wb"))
pickle.dump([],open("emails.dat","wb"))
pickle.dump([],open("passwords.dat","wb"))


#Then replace your program with this


import pickle

Usernames=pickle.load(open('usernames.dat','rb'))
Emails=pickle.load(open('emails.dat','rb'))
Passwords=pickle.load(open('passwords.dat','rb'))


print('Type SU to Sign Up or type LI to Log In!')
UOption = input('Option>> ')

if UOption == 'SU':
    SI = True
    LI = False
    if SI == True:
        print('Signing Up!')
        SUUsername = input('Username>> ')
        SUEmail = input('Email>> ')
        SUPassword = input('Password>> ')

    Usernames.append(SUUsername)
    Emails.append(SUEmail)
    Passwords.append(SUPassword)
    LI = True
    SI = False


pickle.dump(Usernames,open("usernames.dat","wb"))
pickle.dump(Emails,open("emails.dat","wb"))
pickle.dump(Passwords,open("passwords.dat","wb"))