Exercise for reference: 

Create a script that generates a file where all letters of the English alphabet are listed three in each line.

Answer: 

import string

letters = string.ascii_lowercase + " "

slice1 = letters[0::3]
slice2 = letters[1::3]
slice3 = letters[2::3]

with open("letters.txt", "w") as file:
    for s1, s2, s3 in zip(slice1, slice2, slice3):
        file.write(s1 + s2 + s3 + "\n")

Explanation:

This is like the previous exercise with the difference that we're using a different slicing step of 3 here. Another difference is that we are adding one white space to ascii_lowercase  so that we get a string of 27 items instead of 26 that ascii_lowercase  provides. By slicingascii_lowercase we would get two slices with a length of 9 and one slice with a length of 8. 

That means the iteration would stop at the shortest slice, so letters y and z wouldn't be written in the file. That's why we add a white space, so we get three slices, each with a length of 9. That means we get 9 iterations and not 8 as we would get with the original ascii_lowercase  string. The 9th iteration would write letters y and z in the file.

Note that this solution might not be the best, and there are surely improvements to be made, so the script works with any number of letters.