What is the correct way to use **nested list comprehension in Python** to generate combinations of two strings?

Product Support
1

What is the correct way to use nested list comprehension in Python to generate combinations of two strings?

For example, if I have two strings, 'abc' and 'def', I could use two for loops to get all combinations like this:

for j in s1:
    for k in s2:
        print(j, k)

However, I would like to achieve this using nested list comprehension in Python. I’ve tried several attempts, but I haven’t been able to get it working. Can anyone show me how to do this?

2

I’ve worked with Python for a while, and list comprehensions are one of my favorite features. If you’re looking to generate combinations of two strings using nested list comprehensions, here’s a clean and efficient way to do it:

lst = [j + k for j in s1 for k in s2]

If you prefer tuples instead of concatenated strings, modify it like this:

python

CopyEdit

lst = [(j, k) for j in s1 for k in s2]

The for j in s1 part is the outer loop, and for k in s2 is the inner loop. You can stack as many independent for x in y clauses as needed inside a list comprehension.

For better readability, I sometimes spread it across multiple lines like this:

lst = [
    j + k         # result
    for j in s1   # outer loop 
    for k in s2   # inner loop
]

This is a simple and Pythonic way to handle nested loops in a single line!

3

I like using itertools when working with combinations of multiple iterables. Instead of nested list comprehensions, you can also use itertools.product, which directly generates the Cartesian product. It’s a more functional approach and avoids explicit looping. Here’s how you do it:

import itertools

lst = [''.join(p) for p in itertools.product(s1, s2)]

This method is particularly useful when working with more than two strings or when you want to scale your approach to handle different iterable types. itertools.product is efficient and keeps your code clean.

4

If you’re someone who prefers keeping logic inside functions (especially for reusability), you can achieve the same result using traditional nested loops inside a function. It makes the approach more readable and flexible in case additional logic needs to be added later.

def generate_combinations(s1, s2):
    result = []
    for j in s1:
        for k in s2:
            result.append(j + k)
    return result

lst = generate_combinations(s1, s2)

This approach mimics the explicit loop structure and can be particularly useful when you need to debug or extend the logic. Whether to use list comprehensions, itertools, or traditional loops depends on readability, performance, and personal preference.