Interleave multiple lists of the same length in Python

Question

In Python, is there a good way to interleave two lists of the same length?

Say I\'m given [1,2,3] and [10,20,30]. I\'d like to transform th

Answer 1

I like aix's solution best. here is another way I think should work in 2.2:

>>> x=range(3)
>>> x
[0, 1, 2]
>>> y=range(7,10)
>>> y
[7, 8, 9]
>>> sum(zip(x,y),[])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "tuple") to list
>>> sum(map(list,zip(x,y)),[])
[0, 7, 1, 8, 2, 9]

and one more way:

>>> a=[x,y]
>>> [a[i][j] for j in range(3) for i in (0,1)]
[0, 7, 1, 8, 2, 9]

and:

>>> sum((list(i) for i in zip(x,y)),[])
[0, 7, 1, 8, 2, 9]

Answer 2

[el for el in itertools.chain(*itertools.izip_longest([1,2,3], [4,5])) if el is not None]

As long as you don't have None that you want to keep

Answer 3

I needed a way to do this with lists of different sizes which the accepted answer doesn't address.

My solution uses a generator and its usage looks a bit nicer because of it:

def interleave(l1, l2):
    iter1 = iter(l1)
    iter2 = iter(l2)
    while True:
        try:
            if iter1 is not None:
                yield next(iter1)
        except StopIteration:
            iter1 = None
        try:
            if iter2 is not None:
                yield next(iter2)
        except StopIteration:
            iter2 = None
        if iter1 is None and iter2 is None:
            raise StopIteration()

And its usage:

>>> a = [1, 2, 3, 4, 5]
>>> b = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> list(interleave(a, b))
[1, 'a', 2, 'b', 3, 'c', 4, 'd', 5, 'e', 'f', 'g']
>>> list(interleave(b, a))
['a', 1, 'b', 2, 'c', 3, 'd', 4, 'e', 5, 'f', 'g']