Round-robin scheme

Since I already was at improving itertools recipes in the last post, there is a recipe for round-robin consuming of iterables which can be written much more concise and yet — to the functional programmer’s eye — more clearly:

from itertools import chain, izip_longest

def roundrobin(*iterables):
    sentinel = object()
    return (
        x for x in chain(*izip_longest(fillvalue=sentinel, *iterables))
        if x is not sentinel
    )

You also see a recipe for a “sentinel”, that is, a unique value which helps to find the end of a sequence, filter out unneeded values, etc. People tend to use things such as None for this, but this way you never can be sure that this particular None you’re coming across just now is not a non-sentinel. The simplest way to define a unique object is to instantiate an object (sic) and use the is comparison to identify it. A little test-drive confirms that it works as announced:

>>> list(roundrobin('ABC', 'D', 'EF'))
['A', 'D', 'E', 'B', 'F', 'C']

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