为什么“in”生成器比python中的“in”列表快得多

Question

Example:

from timeit import timeit

print(timeit("5 in [i for i in range(0, 100)]"))
print(timeit("5 in map(int, range(0, 100))"))

and this is result:

3.771566713000084
0.9066896029999043

python 3.8.5 (also I think this make no reference to python version ^_^)

Answer 1

The in on map (which is an iterator, not a generator, technically speaking; generators are functions using yield or generator expressions, and they're a subset of the broader class of iterators) short-circuits as soon as it knows the result to be True , so it only actually produces and checks six values and then immediately returns True . The list comprehension, by contrast, must produce the entire list of 100 elements before checking any of them.

If your test was for an element that wasn't in the iterable in question, map 's win, if any (the pointless call to int hurts it, performance-wise), would be smaller, but when the iterable contains the element, and it's early in the iterable, short-circuiting is clearly faster even if each element is more costly to produce, because it produces so many fewer elements.

Answer 2

Generator expressions are much better than list comprehensions.

Iterating over the generator expression or the list comprehension will do the same thing. However, the list comprehension will create the entire list in memory first while the generator expression will create the items on the fly, so you are able to use it for very large and also even infinite sequences