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

If you want the fastest way, you can combine itertools with operator.add:

In [36]: from operator import add

In [37]: from itertools import  starmap, izip

In [38]: timeit "".join([i + j for i, j in uzip(l1, l2)])
1 loops, best of 3: 142 ms per loop

In [39]: timeit "".join(starmap(add, izip(l1,l2)))
1 loops, best of 3: 117 ms per loop

In [40]: timeit "".join(["".join(item) for item in zip(l1, l2)])
1 loops, best of 3: 196 ms per loop

In [41]:  "".join(starmap(add, izip(l1,l2))) ==  "".join([i + j   for i, j in izip(l1, l2)]) ==  "".join(["".join(item) for item in izip(l1, l2)])
Out[42]: True

But combining izip and chain.from_iterable is faster again

In [2]: from itertools import  chain, izip

In [3]: timeit "".join(chain.from_iterable(izip(l1, l2)))
10 loops, best of 3: 98.7 ms per loop

There is also a substantial difference between chain(* and chain.from_iterable(....

In [5]: timeit "".join(chain(*izip(l1, l2)))
1 loops, best of 3: 212 ms per loop

There is no such thing as a generator with join, passing one is always going to be slower as python will first build a list using the content because it does two passes over the data, one to figure out the size needed and one to actually do the join which would not be possible using a generator:

join.h:

 /* Here is the general case.  Do a pre-pass to figure out the total
  * amount of space we'll need (sz), and see whether all arguments are
  * bytes-like.
   */

Also if you have different length strings and you don't want to lose data you can use izip_longest :

In [22]: from itertools import izip_longest    
In [23]: a,b = "hlo","elworld"

In [24]:  "".join(chain.from_iterable(izip_longest(a, b,fillvalue="")))
Out[24]: 'helloworld'

For python 3 it is called zip_longest

But for python2, veedrac's suggestion is by far the fastest:

In [18]: %%timeit
res = bytearray(len(u) * 2)
res[::2] = u
res[1::2] = l
str(res)
   ....: 
100 loops, best of 3: 2.68 ms per loop