I'm building a function that builds a dictionary with words, such as: {'b': ['b', 'bi', 'bir', 'birt', 'birth', 'birthd', 'birthda', 'birthday'], 'bi': ['bi', 'bir', 'birt', 'birth', 'birthd', 'birthda', 'birthday'], 'birt': ['birt', 'birth', 'birthd', 'birthda', 'birthday'], 'birthda': ['birthda', 'birthday'], 'birthday': ['birthday'], 'birth': ['birth', 'birthd', 'birthda', 'birthday'], 'birthd': ['birthd', 'birthda', 'birthday'], 'bir': ['bir', 'birt', 'birth', 'birthd', 'birthda', 'birthday']} This is what it looks like: def add_prefixs(word, prefix_dict): lst=[] for letter in word: n=word.index(letter) if n==0: lst.append(word[0]) else: lst.append(word[0:n]) lst.append(word) lst.remove(lst[0]) for elem in lst: b=lst.index(elem) prefix_dict[elem]=lst[b:] return prefix_dict It works great for words like "birthday", but when I have a letter that repeats itself, I have a problem... for example, "hello". {'h': ['h', 'he', 'he', 'hell', 'hello'], 'hell': ['hell', 'hello'], 'hello': ['hello'], 'he': ['he', 'he', 'hell', 'hello']} I know it's because of the index (python chooses the index of the first time the letter appears) but I do not know how to solve it. Yes, this is my homework and I'm really trying to learn from you guys :)