options1 = ["Yes", "No"]

def options():
    print 'Welcome to my Login System'

    while True:
      option = raw_input("Would you like to view the username list? (Yes/No) : ")

      if option == options1[0]: # Case it's "Yes"
          print "User typed 'Yes'"
          break # Exit the loop
      elif option == options1[1]: # Case it's "No"
          print "User typed 'No'"
          break # Exit the loop
      else: # Case it's anything else
          print "Learn to type!"
          # No break here, ask user again

options()
print "\nProgram ended"