They are different. def square_vals(x : list): return (val * val for val in x) is a function that takes a list and returns a generator. While

They are different. def square_vals(x : list): return (val * val for val in x) is a function that takes a list and returns a generator. While def square_vals(x : list): for val in x: yield val * val is a generator itself. The first case creates an anonymous generator. When in doubt you can always use the dis module: https://docs.python.org/3/library/dis.html https://docs.python.org/3/library/dis.html