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

[Terry Reedy]

On 11/16/2017 2:56 PM, Terry Reedy wrote:

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