Could we have a splice function in itertools? I see there was once a roundrobin proposal (#756253), but it was three years ago ...

Here's an alternate implementation:

def splice(*args):
  """splice(*iterables) --> iterator
  Returns an iterator whose next() method returns an element from each of the iterables in turn before starting again with the first iterable."""
  iters = list(args)
  n = len(iters)
  i = 0
  while n>0:
    i %= n
    try:
      yield iters[i].next()
      i += 1
    except StopIteration, e:
      n -= 1
      iters[i:i+1] = []
  raise StopIteration

The reasons for rejecting the roundrobin proposal still apply three years later.  It is somewhat use case challenged.

FWIW, there is a reasonably efficient pure python implementation using collections.deque().  See http://docs.python.org/lib/deque-recipes.html for an unoptimized recipe which can be fine-tuned to use bound methods and all local variables for better speed.

FWIW, here is the optimized recipe:

def roundrobin(*iterables):
    pending = deque(iter(i).next for i in reversed(iterables))
    rotate, pop, _StopIteration = pending.rotate, pending.pop, StopIteration
    while pending:
        try:
            while 1:
                yield pending[-1]()
                rotate()
        except _StopIteration:
            pop()