Advent of Code Input Parsing

February 22, 2026

I’ve gotten tripped up two years on a row now with parsing this style of input:

3-5
10-14
16-20
12-18

1
5
8
11
17
32

This year it was on day 5. Coincidentally in 2023 it was also on day 5.

I always follow the same line of reasoning - “let’s read in the entire input in as a string and split it into two string “chunks” (the top chunk and the bottom chunk). Then split those chunks into lines.”

import sys
input_str = sys.stdin.read()

ingredient_ranges_str, ingredient_ids_str = input_str.split('\n\n')

ingredient_ranges = ingredient_ranges_str.split('\n')
ingredient_ids = ingredient_ids_str.split('\n')

Seems reasonable enough. The two chunks are separated by two newlines. So first we split on \n\n. And then within those chunks we just split on \n.

Let’s look at the first section:

print(ingredient_ranges)

['3-5', '10-14', '16-20', '12-18']

Looks good! What about the second chunk?

print(ingredient_ids)

['1', '5', '8', '11', '17', '32', '']

Oh no! What is that empty string doing at the end! What happened?

This is about the point where I add a kludge/hack to filter away the trailing empty string so I can get on with solving the problem.

But hold on a second - let’s dig into this and figure out exactly why we’re getting an extra empty string in the second section but not the first!

Let’s print out the two text chunks:

chunks = input.split('\n\n')
print(chunks)

['3-5\n10-14\n16-20\n12-18', '1\n5\n8\n11\n17\n32\n']

Ah ha! We see that the second string chunk ends with a newline, whereas the first one doesn’t. Indeed, all unix files end with a newline.

So we should expect to get an empty string as the last token from str.split().

But hold on a second - why does this normally work? Why don’t we always get an empty string at the end?

Oh, well we are doing str.split('\n'). I usually just do str.split() without the \n. Let’s try that:

ingredient_ids = ingredient_ids_str.split()
print(ingredient_ids)

['1', '5', '8', '11', '17', '32']

Hey, it works! But hold on, why does it work?

Well, it turns out it’s because string.split() with no arguments strips off leading and trailing whitespace first.

I actually had no idea that was the way it worked, which leads to some interesting consequences. For example, out our input file could look like this:


    3-5
10-14
16-20
12-18

1
5
8
11
17
32

And we’ll still get each line separated, exactly how we want:

import sys
input_str = sys.stdin.read()
lines = input_str.split()
print(lines)

['3-5', '10-14', '16-20', '12-18', '1', '5', '8', '11', '17', '32']

Unintuitive, but it gets the job done.

Moral of the story: don’t use string.split('\n') when parsing lines of a file