How do I merge two python iterators?

Question

I have two iterators, a list and an itertools.count object (i.e. an infinite value generator). I would like to merge these two into a resulting ite

Answer 1

Here is an elegant solution:

def alternate(*iterators):
    while len(iterators) > 0:
        try:
            yield next(iterators[0])
            # Move this iterator to the back of the queue
            iterators = iterators[1:] + iterators[:1]
        except StopIteration:
            # Remove this iterator from the queue completely
            iterators = iterators[1:]

Using an actual queue for better performance (as suggested by David):

from collections import deque

def alternate(*iterators):
    queue = deque(iterators)
    while len(queue) > 0:
        iterator = queue.popleft()
        try:
            yield next(iterator)
            queue.append(iterator)
        except StopIteration:
            pass

It works even when some iterators are finite and others are infinite:

from itertools import count

for n in alternate(count(), iter(range(3)), count(100)):
    input(n)

Prints:

0
0
100
1
1
101
2
2
102
3
103
4
104
5
105
6
106

It also correctly stops if/when all iterators have been exhausted.

If you want to handle non-iterator iterables, like lists, you can use

def alternate(*iterables):
    queue = deque(map(iter, iterables))
    ...