Sometimes you need to know the size of a generator without retrieving the actual values. Some generators support len(), but this is not the rule: In : len(range(10000)) Out: 10000 In : gen = (x ** 2 for x in range(10000)) In : len(gen) ... TypeError: object of type 'generator' has no len() The straightforward solution is to use an intermediate list: In : len(list(gen)) Out: 10000 Though fully functional, this solution requires enough memory to store all the yielded values. The simple idiom allows to avoid such a waste: In : sum(1 for _ in gen) Out: 10000