Originally PEP 8 had 79 characters. Now that was a weird choice so most companies went with 80 instead, including Facebook. You want a low-ish limit because it makes it possible to fit two files side by side on a typical screen resolution. Even if you don't edit like that, you look at diffs like that. More importantly, a low column limit is helpful to disabled engineers who don't have to navigate horizontally so much.

So we used 80. I always felt bad when the linter stopped people from pushing impactful changes because they went over the limit by two characters. So two years ago I set up a "highway speed limit" style warning in flake8 (code B950 in PyCQA/flake8-bugbear). What it does is it keeps your limit intact (for example "80") but doesn't trigger unless you went over by more than 10%. So the limit happens to be 88.

When I was working on Black, I was faced with a dilemma. Should the formatter stick to 80 or be able to "go over" a bit, too, as we would let humans do. I felt like the latter made more sense as the resulting code looks nicer (fewer occasions to break a single line into three or more). Then I remembered Raymond's talk "Beyond PEP8" where he mentions that experience shows "90-ish" is the wisest choice. So I went with it.

Science. 88 produced smaller files than 80. And 90 and above didn’t noticeably shorten files.

Like Raymond Hettinger said in his talk beyond pep8, 90ish is better than a strict 80. If you have 81 characters on a line its a waste of time to move that to three lines and now harder to read. So there should be some buffer. that buffer is 10%. The goal is 80 but we are ok with up to 88.

90 characters?!?!? You heathen. I want 132 characters per line.