Using the send() function of a Python generator to approximate lookahead and other uses

regenerator.py

def regenerator(gen):
    for x in gen:
        regen = (yield x)
        while regen is not None:
            yield None  # send() also yields something, so don't pull from gen again
            regen = (yield regen)

gen = (i for i in range(10))
regen = regenerator(gen)
v = next(regen)  # 0
regen.send(v)  # we've seen it, but we want to unsee it; send it back
next(regen)  # still 0
regen.send(-1)  # you don't have to send back the same value
next(regen)  # -1
next(regen)  # 1 (back to where we left off)
regen.send(None)  # yields 2, because None is the same as when send() is not used
next(regen)  # 3

See here: http://code.activestate.com/recipes/528943/