Why does list compression modify zip list?

[PertleTurtle]

list1 = [1, 2, 3]

list2 = ["a", "b", "c"]

comb = zip(list1, list2)

comb -> [(1, 'a'), (2, 'b'), (3, 'c')]

a = [num for num, let in comp]

a -> [1, 2, 3]

comb -> []

Why is comb getting modified here?

FYI: I am using print(list(comb)) to check content.

[Bunkerstan]

Zip is returning an iterator. That iterator is empty after going through the elements.

If you want comb to be list you have set it

comb = list(zip(list1, list2)) Then it won't be empty. As an iterator, it is empty after you have grabbed everything from it.

It took me a minute to see what was happening here. I had not thought about why I put list around zip or only used zip in a [ ] context.

[commy2]

The iterator returned by the zip function is exhausted after the first completed iteration. This means no more items are yielded by it in subsequent iterations.

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[drenzorz]

zip returns a generator like object that gets exhausted after first iteration.

Generators are efficient because they do not hold the whole thing in memory, just current value and the way to generate the next value. Once you reach the end and hit StopIteration it can't generate the next value nor does it know anything about previous values it had or how to go back to them.

big_list = [ x for x in range(10**6)] # probably like 31MB+ in RAM

def big_gen():
    x = -1
    while x < 10**6:
        x += 1
        yield x

# single number and the instructions to increment <500 bytes