python: most elegant way to intersperse a list with an element

Another option that works for sequences:

def intersperse(seq, value):
    res = [value] * (2 * len(seq) - 1)
    res[::2] = seq
    return res

itertools to the rescue
- or -
How many itertools functions can you use in one line?

from itertools import chain, izip, repeat, islice

def intersperse(delimiter, seq):
    return islice(chain.from_iterable(izip(repeat(delimiter), seq)), 1, None)

Usage:

>>> list(intersperse(666, ["once", "upon", "a", 90, None, "time"])
["once", 666, "upon", 666, "a", 666, 90, 666, None, 666, "time"]

I would have written a generator myself, but like this:

def joinit(iterable, delimiter):
    it = iter(iterable)
    yield next(it)
    for x in it:
        yield delimiter
        yield x