traverse iterable with prev and next context

traverse_prev_next.py

from collections.abc import Iterable, Iterator
from typing import TypeVar

T = TypeVar('T')


def _scan(items: Iterable[T]) -> Iterator[tuple[T, T]]:

    """iterator to traverse previous and current items from iterable"""
    prev, curr, stream = None, None, iter(items)
    while True:
        try:
            prev = curr
            curr = next(stream)
            yield prev, curr
        except StopIteration:
            break


def scan(items: Iterable[T]) -> Iterator[tuple[T, T, T]]:
    """iterator to traverse previous, current, and next items from iterable"""
    last, stream = None, _scan(_scan(items))
    try:
        first = next(stream)
    except StopIteration:
        return
    for x, y in stream:
        yield x[0], x[1], y[1]
        last = y
    if last:
        yield last[0], last[1], None
    else:
        yield None, first[1][1], None


if __name__ == '__main__':
    test_input = iter('abc')
    for trace in scan(test_input):
        print(trace)