Pythonic way to get n elements at time from list

There are similar questions but i didn't find a clean answer for what i think is simple.

I want to get once the first 1000 elements of a list, then the first 2000 ecc...

for i in range(1000, len(l1), 1000):
     a = l1[:i]

but obviously won't work if len(l1) % 1000 != 0 or if len(l1) < 1000. What is an easy solution for this?

Thanks.