[Python-ideas] Rewriting the "roundrobin" recipe in the itertools documentation

[Serhiy Storchaka]

The roundrobin() implementation in recipes has quadratic time for large 
number of iterables. As well as all other proposed implementations. This 
is a problem if you use it with hundreds or thousands of iterables. For 
example:

     list(roundrobin(*([[1]]*1000)))
     next(roundrobin(*([[]]*1000 + [[1]]])))

The following implementation has linear time.

def roundrobin(*iterables):
     "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
     nexts = [iter(it).__next__ for it in iterables]
     while nexts:
         next_nexts = []
         append = next_nexts.append
         for next in nexts:
             try:
                 yield next()
             except StopIteration:
                 pass
             else:
                 append(next)
         nexts = next_nexts

Actually it expands cycle() in Python. And this makes it slower for 
smaller number of iterations.