from operator import gt, lt


def my_range_gen(start, stop=None, step=1):
    if stop is None:
        start, stop = 0, start

    if (
        not step
        or start == stop
        or (step > 0 and start > stop)
        or (step < 0 and start < stop)
    ):
        return

    x, op = start, start > stop and gt or lt
    while op(x, stop):
        yield x
        x += step


for i in my_range_gen(20, 10, 3):
    print(i)
print("End")