Joining on a list (as in function two()) is no more memory intensive than joining on a generator expression (function one()). This is surprising at first. Raymo

Joining on a list (as in function two()) is no more memory intensive than joining on a generator expression (function one()). This is surprising at first. Raymond Hettinger has a very good explanation.[0] In short, it's because "".join() needs to iterate over the elements twice, and so it needs to store the whole sequence of string anyway. [0] https://stackoverflow.com/a/9061024 https://stackoverflow.com/a/9061024