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