list comprehension filtering - “the set() trap”

Question

A reasonably common operation is to filter one list based on another list. People quickly find that this:

[x for x in list_1 if x          


        

Answer 1

In order to optimize set(list_2), the interpreter needs to prove that list_2 (and all of its elements) does not change between iterations. This is a hard problem in the general case, and it would not surprise me if the interpreter does not even attempt to tackle it.

On the other hand a set literal cannot change its value between iterations, so the optimization is known to be safe.