Pythonic way to combine two lists in an alternating fashion?

Question

I have two lists, the first of which is guaranteed to contain exactly one more item than the second. I would like to know the most Pythonic way to create a

Answer 1

Here's a one liner using list comprehensions, w/o other libraries:

list3 = [sub[i] for i in range(len(list2)) for sub in [list1, list2]] + [list1[-1]]

Here is another approach, if you allow alteration of your initial list1 by side effect:

[list1.insert((i+1)*2-1, list2[i]) for i in range(len(list2))]

Answer 2

This is nasty but works no matter the size of the lists:

list3 = [element for element in list(itertools.chain.from_iterable([val for val in itertools.izip_longest(list1, list2)])) if element != None]

Answer 3

There's a recipe for this in the itertools documentation:

from itertools import cycle, islice

def roundrobin(*iterables):
    "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
    # Recipe credited to George Sakkis
    pending = len(iterables)
    nexts = cycle(iter(it).next for it in iterables)
    while pending:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            pending -= 1
            nexts = cycle(islice(nexts, pending))

EDIT:

For python's version greater than 3:

from itertools import cycle, islice

def roundrobin(*iterables):
    "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
    # Recipe credited to George Sakkis
    pending = len(iterables)
    nexts = cycle(iter(it).__next__ for it in iterables)
    while pending:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            pending -= 1
            nexts = cycle(islice(nexts, pending))

Answer 4

If both lists have equal length, you can do:

[x for y in zip(list1, list2) for x in y]

As the first list has one more element, you can add it post hoc:

[x for y in zip(list1, list2) for x in y] + [list1[-1]]

Answer 5

I'd do the simple:

chain.from_iterable( izip( list1, list2 ) )

It'll come up with an iterator without creating any additional storage needs.

Answer 6

import itertools
print [x for x in itertools.chain.from_iterable(itertools.izip_longest(list1,list2)) if x]

I think this is the most pythonic way of doing it.