Split iterator into multiple streams

[Steven D'Aprano]

Suppose I have an iterator that yields tuples of N items (a, b, ... n).

I want to split this into N independent iterators:

iter1 -> a, a2, a3, ...
iter2 -> b, b2, b3, ...
...
iterN -> n, n2, n3, ...

The iterator may be infinite, or at least too big to collect in a list.

My first attempt was this:


def split(iterable, n):
    iterators = []
    for i, iterator in enumerate(itertools.tee(iterable, n)):
        iterators.append((t[i] for t in iterator))
    return tuple(iterators)

But it doesn't work, as all the iterators see the same values:

>>> data = [(1,2,3), (4,5,6), (7,8,9)]
>>> a, b, c = split(data, 3)
>>> list(a), list(b), list(c)
([3, 6, 9], [3, 6, 9], [3, 6, 9])


I tried changing the t[i] to use operator.itergetter instead, but no 
luck. Finally I got this:

def split(iterable, n):
    iterators = []
    for i, iterator in enumerate(itertools.tee(iterable, n)):
        f = lambda it, i=i: (t[i] for t in it)
        iterators.append(f(iterator))
    return tuple(iterators)

which seems to work:

>>> data = [(1,2,3), (4,5,6), (7,8,9)]
>>> a, b, c = split(data, 3)
>>> list(a), list(b), list(c)
([1, 4, 7], [2, 5, 8], [3, 6, 9])




Is this the right approach, or have I missed something obvious?



-- 
Steven