存储大列表的前X个项目（Python）

Question

Let's say we generate a large list of lists in Python using the combinations iterable, and you wanted to retrieve a list with certain properties.

If the list of lists is sufficiently large, merely generating the list will take the bulk of the calculation time.

Is there any way of generating the first x members of the iterable to reduce the overall calculation time? Let's say:

y=50
z=[list(combo) for combo in combinations(range(y + 1), 11)]

Let's say we have a function A, such that when we apply A to z, we get the required lists quickly. Is there a method for applying the function A after each member of z is calculated? So instead of calculating the entire list of lists first, the instant a single item is created, the function evaluates it?

More generally, is there any method for retrieving the first x members of z, and not the entire list of lists?

Answer 1

Yup, use itertools.islice to limit the number.

from itertools import islice

combos = combinations(range(y + 1), 11)
data = [list(combo) for combo in islice(combos, 100)]

To take the first 100 from the combinations of combos... You can then keep consuming from combos afterwards...

print next(combos) # next combination
print list(islice(combos, 3, 13)) # skip 3 and take another 10