Lookup time for dict() is O(1) and for list() is O(n) no matter what the number of elements is. Given a list of elements in a file, this piece of code: from itertools import izip lists = map(str.strip, open(‘dict.txt’).readlines()) i = iter(lists) j = range(len(lists)) dicts = dict(izip(i, j)) index = dicts[w] is n time faster than this piece of code: dicts = map(str.strip, open(‘dict.txt’).readlines()) index = dicts.index(w) I learned that the hard way. My dict has 5,000,000 elements, which mean my code will run 5 millions time slower than it should be. As such, I ran my code on 20 machines, each with 64 cores and it took ~10 hours without finish. Changing from list to dict, it took about 900 seconds on a single machine (of course 64 cores) Next time, when you want to do lookup in Python, use dict(). Happy coding!