Python Tail of Iterable from https://twitter.com/Wildvasa/status/1207383049387810816

iteratable_tail_example.py

#!/usr/bin/env python3
from collections import deque


def to_tail(it, max_items):
    """

    https://twitter.com/Wildvasa/status/1207383049387810816

    Get the last N elements from an iterable

    This modification will avoid reading in the
    entire iterable into memory if possible.
    """

    # this will sort out the impedance mismatch
    ix = iter(it)

    dq = deque(maxlen=max_items)

    # this needs to be tuned to
    # clarify the expected behavior
    # when the max_items is longer
    # than the iterable and when the
    # iterable is empty
    for i in range(0, max_items):
        try:
            item = next(ix)
            dq.append(item)
        except StopIteration:
            pass

    for i in ix:
        # O(1)
        dq.popleft()
        dq.append(i)

    for x in dq:
        yield x


def test_to_tail():

    def to_gen(max_items):
        for i in range(0, max_items):
            yield i * i

    def run(it, max_items):
        print("running it={} type={} max-items={}".format(it, type(it), max_items))
        print(list(to_tail(it, max_items)))
        print("")

    tests = [
        (range(0, 10), 3),
        (range(0, 20), 5),
        (to_gen(10), 3),
        (to_gen(10), 15),
        (to_gen(20), 2),
        ([1, 2, 3], 1),
        ('abcd', 3)
    ]

    for it, max_items in tests:
        run(it, max_items)


if __name__ == '__main__':
    test_to_tail()