177 – nwise

The iterable itertools.pairwise can be generalised to create an iterable that produces overlapping tuples of arbitrary length, instead of just pairs.

One of the best ways to generalise pairwise is by using collections.deque to manage the window of values that is going to be produced next:

from collections import deque
from itertools import islice

def nwise(values, n):
    iterator = iter(values)
    window = deque(islice(iterator, n - 1), maxlen=n)
    for value in iterator:
        window.append(value)
        yield tuple(window)

The line window = deque(islice(...), ...) seeds the deque with its first values. Then, the loop adds a value to the window and yields it as a tuple. deque is convenient to use here because of its parameter maxlen, that automatically makes sure the deque doesn’t grow too large.