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

You could also do this using map and operator.add:

from operator import add

u = 'AAAAA'
l = 'aaaaa'

s = "".join(map(add, u, l))

Output:

'AaAaAaAaAa'

What map does is it takes every element from the first iterable u and the first elements from the second iterable l and applies the function supplied as the first argument add. Then join just joins them.

Answer 2

Faster Alternative

Another way:

res = [''] * len(u) * 2
res[::2] = u
res[1::2] = l
print(''.join(res))

Output:

'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz'

Speed

Looks like it is faster:

%%timeit
res = [''] * len(u) * 2
res[::2] = u
res[1::2] = l
''.join(res)

100000 loops, best of 3: 4.75 µs per loop

than the fastest solution so far:

%timeit "".join(list(chain.from_iterable(zip(u, l))))

100000 loops, best of 3: 6.52 µs per loop

Also for the larger strings:

l1 = 'A' * 1000000; l2 = 'a' * 1000000

%timeit "".join(list(chain.from_iterable(zip(l1, l2))))
1 loops, best of 3: 151 ms per loop


%%timeit
res = [''] * len(l1) * 2
res[::2] = l1
res[1::2] = l2
''.join(res)

10 loops, best of 3: 92 ms per loop

Python 3.5.1.

Variation for strings with different lengths

u = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
l = 'abcdefghijkl'

Shorter one determines length (zip() equivalent)

min_len = min(len(u), len(l))
res = [''] * min_len * 2 
res[::2] = u[:min_len]
res[1::2] = l[:min_len]
print(''.join(res))

Output:

AaBbCcDdEeFfGgHhIiJjKkLl

Longer one determines length (itertools.zip_longest(fillvalue='') equivalent)

min_len = min(len(u), len(l))
res = [''] * min_len * 2 
res[::2] = u[:min_len]
res[1::2] = l[:min_len]
res += u[min_len:] + l[min_len:]
print(''.join(res))

Output:

AaBbCcDdEeFfGgHhIiJjKkLlMNOPQRSTUVWXYZ

Answer 3

With join() and zip().

>>> ''.join(''.join(item) for item in zip(u,l))
'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz'

Answer 4

Feels a bit un-pythonic not to consider the double-list-comprehension answer here, to handle n string with O(1) effort:

"".join(c for cs in itertools.zip_longest(*all_strings) for c in cs)

where all_strings is a list of the strings you want to interleave. In your case, all_strings = [u, l]. A full use example would look like this:

import itertools
a = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
b = 'abcdefghijklmnopqrstuvwxyz'
all_strings = [a,b]
interleaved = "".join(c for cs in itertools.zip_longest(*all_strings) for c in cs)
print(interleaved)
# 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz'

Like many answers, fastest? Probably not, but simple and flexible. Also, without too much added complexity, this is slightly faster than the accepted answer (in general, string addition is a bit slow in python):

In [7]: l1 = 'A' * 1000000; l2 = 'a' * 1000000;

In [8]: %timeit "".join(a + b for i, j in zip(l1, l2))
1 loops, best of 3: 227 ms per loop

In [9]: %timeit "".join(c for cs in zip(*(l1, l2)) for c in cs)
1 loops, best of 3: 198 ms per loop

Answer 5

Jim's answer is great, but here's my favorite option, if you don't mind a couple of imports:

from functools import reduce
from operator import add

reduce(add, map(add, u, l))

Answer 6

I like using two fors, the variable names can give a hint/reminder to what is going on:

"".join(char for pair in zip(u,l) for char in pair)