def login(user, password):
    '''Return True iff the login is successful using credentials (user,
    password). 
    
    Return False if the login is not successful, either because of wrong
    credentials or because the user was locked out. Print a message if
    a locking out occurs.
    
    Locking out occurs if three attempts in a row are made to log in with
    invalid credentials. 
    
    The credentials are stored in the global variables usernames and passwords,
    with credential pairs being users[i], passwords[i].
    
    Modify the global n_attempts to keep track of attempts to login with 
    invalid credentials that occur one after each other.
    
    Arguments:
    user -- a string
    password -- a string
    '''
    global unsuccessful_attempts
    #can use users passwords
    if unsuccessful_attempts >= 3:
        return False
    
    if user not in users: 
        unsuccessful_attempts += 1
        if unsuccessful_attempts >= 3:
            print("You're locked out now!")
        return False
        
    
                
    if passwords[users.index(user)] == password:
        unsuccessful_attempts = 0
        return True
    else:
        unsuccessful_attempts += 1
        if unsuccessful_attempts >= 3:
            print("You're locked out now!")
        return False
        
        
    
def initialize():
    '''Set the globals variables users and passwords, and 
    initialize the global unsuccessful_attempts to 0
    '''
    global users, passwords, unsuccessful_attempts
    
    #usernames and passwords. users[i], passwords[i]
    #are matching pairs
    users = ["foster", "collins", "thywissen"]
    passwords = ["praxis!", "elegance", "midterm"]
    
    #The current number of consecutive unsuccessful attempts
    unsuccessful_attempts = 0

initialize()

if __name__ == '__main__':
    #Test case 1: a lock out should occur.
    initialize()
    login("foster", "praxis!")
    login("guerzhoy", "a234fsadkjl")
    
    login("guerzhoy", "a234fsadkjl")
    login("guerzhoy", "a234fsadkjl")
    
    if login("thywissen", "midterm") == False:
        print("Test 1 passed")
    else:
        print("Test 1 failed")
    
    #Test case 2: a lock out should not occur: 3 failed
    #logins, but with one succesful login in between
    initialize()
    login("guerzhoy", "a234fsadkjl")
    login("foster", "praxis!")
    login("guerzhoy", "a234fsadkjl")
    login("guerzhoy", "a234fsadkjl")
    
    if login("thywissen", "midterm") == True:
        print("Test 2 passed")
    else:
        print("Test 2 failed")

You're locked out now!
Test 1 passed
Test 2 passed

Idea: looking up the password

In order to look up the password for user user, we need to figure out the i such that users[i] == user. Then, we can use that information to look up passwords[i].

users.index(user) gives us that i. We then can use passwords[users.index(user)] in order to get the password associated with the user.

Idea: keep track of failed attempts using a global variable

We set up a global variable called unsuccessful_attempts. The function login modifies it as appropriate (setting it to 0 when there is a successful attempt, increasing it when there is an unsuccesful attempt.) Basically, login() makes sure that unsuccessful_attempts always contains the current number of consecutive unsuccessful login attempts. That way, login() can return False when unsuccessful_attempts is 3.