How does python optimize conditional list comprehensions

Question

I read about List comprehension without [ ] in Python so now I know that

\'\'.join([str(x) for x in mylist])

is faster than

\'\         


        

Answer 1

List comprehensions don't pre-size the list, even when they totally could. You're assuming the presence of an optimization that isn't actually done.

The list comprehension is faster because all the iterator machinery and the work of entering and exiting the genexp stack frame has a cost. The list comprehension doesn't need to pay that cost.