userDatabase = {}
validation = True
while validation == True:
    print ("Create a username. Or, type 'cancel' to cancel.") #Why does it still print this after a
password passes validation and is added to the dictionary?
    username = input()
    if username == 'cancel':
        break
    if username.isalnum() == True:
        if username in userDatabase.keys():
            print ()
            print ("***Error***: Username not available.")
            print ()
        else:
            userDatabase.setdefault(username,"")
            print ("Now please create a password")
            while True:
                password = input()
                if password.isalnum() == True:
                    userDatabase[username] = password
                    validation == False
                    break
                else:
                    print ("Please enter a valid password using letters and numbers only.")
    else:
        print ()
        print ("Error: Invalid.")
        print ()

print(userDatabase)