How can we tell if we are at the beginning, at the end, or within a generated sequence in python?

first_last_inner_items_of_sequence.py

'''
From the @NumFOCUS telethon

title: Z for zip
author: @dontusethiscode
date: December 19, 2020
python version: >= 3.8
video: https://youtu.be/gzbmLeaM8gs?t=15648
edited by: @vitalizzare
date: January 10, 2021
'''

# Suppose different operations are to be performed on the first,
# last and inner generated elements. How can we tell if we are
# at the beginning, at the end, or within a sequence?

from itertools import islice, tee, repeat, chain, zip_longest


def first(g, n=1):
    '''Mark first n items of a sequence g'''
    marker = chain(repeat(True, n), repeat(False))
    for mark, item in zip(marker, g):
        yield mark, item


def nwise_longest(g, n=2, fv=object()):
    '''Return a view on n sequentional elements of a sequence g'''
    # Get a sequence of shifted generators g[i:] for i in range(n)
    shifted = (islice(g, idx, None) for idx, g in enumerate(tee(g, n)))
    for n_slice in zip_longest(*shifted, fillvalue=fv):
        yield n_slice


def last(g, n=1, sentinel=object()):
    '''Mark last n items of a sequence g'''
    for x, *y in nwise_longest(g, n+1, sentinel):
        yield y[-1] is sentinel, x


seq = 'abcd'
print(f'{seq = }')
for is_first, (is_last, item) in first(last(seq)):
    if is_first:
        print('START')
    print('...', item)
    if is_last:
        print('STOP')