Python How to prevent input same value in a empty list for several times : **Wrong** example

a = [ ]

while  a [ 4 ] ! = None: #ensure in put 5 times

        option = input()

        if option in a: #prevent input same value

            continue  
        else:
            a.append(option)

Moreover, if i use dict is better? 

And I am not native of English 