Interleave two strings in Python

September 7, 2015

One programming interview question I've seen is: "Write a function to interleave two strings. You should takes two strings as input and returns one string as output. For example if the input strings are pete and paul then the output should be ppeatuel".

I think it's a good question as it's a little bit tricker than it initially seams. The candidate needs to dig into the problem a further by checking what happens when the strings are of different length. Sensible behavior would be to add the extra characters to the end of the return string, if our input strings are pete graham and paul then the output should be ppeatuel graham.

When I first saw this question my initial solution was to find the length of the larger of the strings. Then loop from the start to the end of the largest string constructing the return string:

def interleave_strings(string1, string2):
    max_length = max(len(string1), len(string2))
    return_string = ''
    for count in range(0, max_length):
        char1 = get_char(string1, count)
        char2 = get_char(string2, count)
        return_string += '{0}{1}'.format(char1, char2)
    return return_string

A function get_char is needed to return an empty string if the index is out of bounds:

def get_char(string, count):
    try:
        char = string[count]
    except IndexError:
        char = ''
    return char

This solution is easy to follow but fairly verbose. Rob Berry pointed this can be done much more succinctly in Python using zip:

def interleave_strings(string1, string2):
    return list(zip(string1, string1))

We need to cast the return value to a list in Python 3 or we'll be returning a zip object. Calling the function with the inputs pete and paul gives us a list of tuples:

[('p', 'p'), ('e', 'a'), ('t', 'u'), ('e', 'l')]

The data structure just needs to be flattened to get our desired result. However there is a flaw in this solution, to work correctly the strings must be the same length. Using pete graham and paul as the input our output is:

[('p', 'p'), ('e', 'a'), ('t', 'u'), ('e', 'l')]

Not good! Our longer string has been truncated.

This can be solved by replacing zip with zip_longest from the itertools module and providing a fill value of an empty string. Note: zip_longest was called izip_longest in Python 2.

from itertools import zip_longest

def interleave_strings(string1, string2):
    return list(zip_longest(string1, string2, fillvalue=''))

We have to cast our return value to a list otherwise an itertools.zip_longest object will be returned. Using the input values pete graham and paul we now get the following result:

[('p', 'p'), ('e', 'a'), ('t', 'u'), ('e', 'l'), (' ', ''), ('g', ''), ('r', ''), ('a', ''), ('h', ''), ('a', ''), ('m', '')]

Now we need to flatten the data structure. One solution is to force unpacking with * and then use chain from itertools to combine the iterators:

from itertools import chain, zip_longest

def interleave_strings(string1, string2):
    return list(chain(*zip_longest(string1, string2, fillvalue='')))

Using our inputs pete graham and paul this gives us a list as output:

['p', 'p', 'e', 'a', 't', 'u', 'e', 'l', ' ', '', 'g', '', 'r', '', 'a', '', 'h', '', 'a', '', 'm', '']

The final piece of the puzzle is to join the list items to return a string:

from itertools import chain, zip_longest

def interleave_strings(string1, string2):
    return ''.join(chain(*zip_longest(string1, string2, fillvalue='')))

This returns the string ppeatuel graham. Wooh!

The above code gives us our desired output, however it can be argued it looks cryptic. An alternative solution is to replace the use of chain with a longer hand solution that uses list comprehension:

from itertools import zip_longest

def interleave_strings(string1, string2):
    tuples = zip_longest(string1, string2, fillvalue='')
    string_list = [''.join(item) for item in tuples]
    return ''.join(string_list)

I've covered three ways to solve the problem, they are many more which can be found on Stack Overflow. The fact the question can be answered in a number of different ways is part of what I think makes it a good one.

If you'd lke to contact me about this article then use twitter @petexgraham or email articles@petegraham.co.uk.