A normal generator/iterator that additionally keeps track of the previous element in Python. Also includes an iterator to keep track of next (after) element as well.

previous_current_iterator.py

from itertools import tee, chain

# ABC -> (None, A) (A, B) (B, C)
def prev_curr_iter(iterable):
    a, b = tee(iterable)
    return zip(chain([None], a), b)
  
# use case
for prev, curr in prev_curr_iter("ABC"):
    print(f"{prev=}\t{curr=}")

# -------------

# ABC -> (None, A, B) (A, B, C) (B, C, None)
def prev_curr_after_iter(iterable):
    a, b, c = tee(iterable, 3)
    next(c, None)
    return zip(chain([None], a), b, chain(c, [None]))

# use case
for prev, curr, after in prev_curr_after_iter("ABCDEFG"):
    print(f"{prev=}\t{curr=}\t{after=}")