On 11/20/2017 11:08 AM, Steven D'Aprano wrote:

Please don't make claims about correctness and efficiency without
testing the code first. The second suggestion given there, using deque,
is *not* correct as provided, as it fails to work with iterables. It
requires the caller to pass only iterators, unlike the existing
roundrobin recipe which accepts any iterable.

Nor is it more efficient, at least on my machine -- in fact the
opposite, it is the worst performing of the four recipes I've tried:

- the current recipe from the itertools docs;
- your re-write, using zip_longest;
- Terry's version;
- and the one from stackoverflow.

I've attached my test code, in case you want to play around with it.
Apologies in advance for any bugs in the test code (its 2 in the
morning here and I've had a long day).

According to my testing, on my computer using Python 3.5, Terry's code
is by far the fastest in all three separate test cases, but that
probably shouldn't count since it's buggy (it truncates the results and
bails out early under some circumstances). Out of the implementations
that don't truncate, the existing recipe is by far the fastest.

Terry, if you're reading this, try:

list(roundrobin('A', 'B', 'CDE'))

Your version truncates the results to A B C instead of A B C D E as the
itertools recipe gives.

This is due to an off-by-1 error which I corrected 3 hours later in a follow-up post, repeated below.
---

Correct off-by-one error.  I should have tested with an edge case such as
print(list(roundrobin('ABC', '')))

> The following combines 3 statements into one for statement.
>
> def roundrobin(*iterables):
>      "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
>      nexts = cycle(iter(it).__next__ for it in iterables)
>      for reduced_len in reversed(range(1, len(iterables))):

Make that 0 rather than 1 for start value.

>          try:
>              for next in nexts:
>                  yield next()
>          except StopIteration:
>              nexts = cycle(islice(nexts, reduced_len))

A slightly clearer, slightly less efficient alternative would be

def roundrobin(*iterables):
    "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
    nexts = cycle(iter(it).__next__ for it in iterables)
    for current_len in reversed(range(1, len(iterables)+1)):
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            nexts = cycle(islice(nexts, current_len - 1))

--
Terry Jan Reedy

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