Consuming two iterators in parallel

Question

Suppose I have two iterators, and I want to compute

fancyoperation1(iter1), fancyoperation2(iter2)

Normally, I would simply use fancyoper

Answer 1

You absolutely can use coroutines for this, it's just slightly less convenient (but on the bright side, you can keep them separated and can leave most code unaltered). Change the fancy operations to be parameterless and repeatedly use yield (as expression) to fetch data instead of accepting a parameter and iterating over it. In other words, change this:

def fancyoperation1(it):
    for x in it:
        ...
    cleanup()

# into something like this

def fancyoperation1():
    while True:
        try:
            x = yield
        except GeneratorExit:
            break
        ...
    cleanup()

Of course, it's easier if there is no post-iteration clean up to be done. You can use these like this (assuming iter1, iter2 = tee(underlying_iter)):

f1, f2 = fancyoperation1(), fancyoperation2()
f1.send(None) # start coroutines
f2.send(None)

for x in underlying_iterator:
    f1.send(x)
    f2.send(x)
f1.close()
f2.close()