Most pythonic way to interleave two strings

Question

What\'s the most pythonic way to mesh two strings together?

For example:

Input:

u = \'ABCDEFGHIJKLMNOPQRSTUVWXYZ\'
l = \'abcdefghijklmnopqrst         


        

Answer 1

Potentially faster and shorter than the current leading solution:

from itertools import chain

u = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
l = 'abcdefghijklmnopqrstuvwxyz'

res = "".join(chain(*zip(u, l)))

Strategy speed-wise is to do as much at the C-level as possible. Same zip_longest() fix for uneven strings and it would be coming out of the same module as chain() so can't ding me too many points there!

Other solutions I came up with along the way:

res = "".join(u[x] + l[x] for x in range(len(u)))

res = "".join(k + l[i] for i, k in enumerate(u))

Answer 2

On Python 2, by far the faster way to do things, at ~3x the speed of list slicing for small strings and ~30x for long ones, is

res = bytearray(len(u) * 2)
res[::2] = u
res[1::2] = l
str(res)

This wouldn't work on Python 3, though. You could implement something like

res = bytearray(len(u) * 2)
res[::2] = u.encode("ascii")
res[1::2] = l.encode("ascii")
res.decode("ascii")

but by then you've already lost the gains over list slicing for small strings (it's still 20x the speed for long strings) and this doesn't even work for non-ASCII characters yet.

FWIW, if you are doing this on massive strings and need every cycle, and for some reason have to use Python strings... here's how to do it:

res = bytearray(len(u) * 4 * 2)

u_utf32 = u.encode("utf_32_be")
res[0::8] = u_utf32[0::4]
res[1::8] = u_utf32[1::4]
res[2::8] = u_utf32[2::4]
res[3::8] = u_utf32[3::4]

l_utf32 = l.encode("utf_32_be")
res[4::8] = l_utf32[0::4]
res[5::8] = l_utf32[1::4]
res[6::8] = l_utf32[2::4]
res[7::8] = l_utf32[3::4]

res.decode("utf_32_be")

Special-casing the common case of smaller types will help too. FWIW, this is only 3x the speed of list slicing for long strings and a factor of 4 to 5 slower for small strings.

Either way I prefer the join solutions, but since timings were mentioned elsewhere I thought I might as well join in.