list comprehension filtering – “the set() trap”

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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.

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