Clarification on PEP 8 E501: line too long (> 79 characters)

[nrogers64]

From ​the "Coding style" documentation:

One big exception to PEP 8 is our preference of longer line lengths. We’re well into the 21st Century, and we have high-resolution computer screens that can fit way more than 79 characters on a screen. Don’t limit lines of code to 79 characters if it means the code looks significantly uglier or is harder to read.

I have two issues with this:

1. It says that you don't have a limit of 79 characters, but it doesn't say what you do instead. Is a line that is 1000+ characters acceptable?
2. It seems to imply that this PEP 8 rule is outdated and that there is absolutely no reason whatsoever that anybody should follow it in a Django project these days. It's even worded in kind of a condescending manner, in my opinion. However, many developers prefer to have multiple files open side by side. For example, they might have a models.py and views.py file open side by side, or even multiple views of the same file open side by side. The limit of 79 characters is helpful here. I would suggest taking out the "21st Century" bit which seems to be there to justify Django not following this PEP 8 rule, but, as I've pointed out, it doesn't really justify it.

[Wim Feijen]

Hi nrogers64,

I agree with you that this text needs updating. As far as I know, the current guideline is actually to keep lines within 79 characters unless it doesn't fit, in which case it is ok to make longer lines. Clarity goes above following the character limit.

Maybe just delete that one sentence and make it:
One exception to PEP 8 is our opinion on line length. Don’t limit lines of code to 79 characters if it means the code looks significantly uglier or is harder to read.

[Baptiste Mispelon]

I agree that the tone is too snarky and FYI, pep8 was recently amended regarding line lengths:

Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the nominal line length from 80 to 100 characters (effectively increasing the maximum length to 99 characters), provided that comments and docstrings are still wrapped at 72 characters.

(​http://legacy.python.org/dev/peps/pep-0008/#maximum-line-length)

[Tim Graham]

Perhaps outside the scope of this ticket, but I'd like to propose adopting a max line length of 120 as that's the width of code review in GitHub. Anything longer requires horizontal scrolling which makes review more difficult. We could use flake8 to enforce this, however, it'll require a lot of cleanup as we currently have ~1700 lines longer than that. I'll attach an initial patch to show how long strings can be restructured.

[Collin Anderson]

I use 120 (119) characters personally, again, because of github.

[Daniele Procida]

I've marked this ticket as especially suitable for people following the ​Don't be afraid to commit tutorial at the DjangoCon US 2014 sprints. If you're tackling this ticket, please don't hesitate to ask me for guidance if you'd like any, either here or on the Django IRC channels, where I can be found as EvilDMP.

[Daniele Procida]

See ​https://groups.google.com/d/msgid/django-developers/8772de23-2970-4b60-91ae-f5817f9b2bac%40googlegroups.com.

[Tim Graham]

I'm working on the updates in django/ and I've split up the test updates into 3 files. Please claim one by commenting here if you want to work on it.

[nrogers64]

Tim, the only issue I have with your proposed wording at django-developers is this:

Documentation, comments, and docstrings should be wrapped at 79 characters.

As stated by bmispelon, PEP 8 says:

For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the nominal line length from 80 to 100 characters (effectively increasing the maximum length to 99 characters), provided that comments and docstrings are still wrapped at 72 characters.

Also, I'm not sure if Carl is correct or not in this statement that he made:

With recent updates to PEP 8 to accommodate longer lines, the stated policy is no longer an exception to PEP 8

The quote from PEP 8 shown above seems to imply that you can go up to 99 characters if you want to, but no more than that. Maybe I'm misinterpreting that, though.

[Carl Meyer]

Replying to nrogers64:

Also, I'm not sure if Carl is correct or not in this statement that he made:

With recent updates to PEP 8 to accommodate longer lines, the stated policy is no longer an exception to PEP 8

The quote from PEP 8 shown above seems to imply that you can go up to 99 characters if you want to, but no more than that. Maybe I'm misinterpreting that, though.

You're right; I didn't re-check the PEP and mis-remembered it as allowing up to 120, not 100. So allowing up to 120 remains an exception to PEP 8. Thanks for the correction.

[Tim Graham]

I've updated the proposed wording in ​this PR which also fixes long lines in django/.

[Tim Graham <timograham@…>]

In 1101467ce0756272a54f4c7bc65c4c335a94111b:

Limited lines to 119 characters in django/

refs #23395.

[conor]

Hi guys, first-time contributor here. Am I right in saying that this patch has been applied and that this ticket can be closed? Sorry if I'm missing something.

[Baptiste Mispelon]

There are still some lines longer than 120 characters, that's why this ticket still hasn't been closed.

You can find those lines by running flake8 --select=E501 (you need to install the flake8 package first).

[Berker Peksag]

​https://github.com/django/django/pull/3421

I left two instances untouched:

• django/db/migrations/loader.py:115:120: E501 line too long (133 > 119 characters)
• django/db/migrations/autodetector.py:432:120: E501 line too long (166 > 119 characters)

[Tim Graham <timograham@…>]

In c9178ef17a0e1070f5a25096d8e13385d404dc92:

Limited lines to 119 characters in django/{contrib,db}.

Refs #23395.

[Tim Graham <timograham@…>]

In d73c7e5db66b7c149db59e07bc4c6253629cf907:

[1.7.x] Limited lines to 119 characters in django/{contrib,db}.

Refs #23395.

Backport of c9178ef17a (to decrease chance of backport conflicts) from master

[Amine Zyad]

I'll take care of this.

[Tim Graham]

Doesn't seem much value here in addressing the line lengths unless we can also exclude migration directories from being checked because the auto-generated code there often has very long lines. Excluding */migrations/* as done before 9d30412a5ad1c72b3d319b4c2bceacb53a0ff1da, however, prevents us from detecting other issues, especially in django/db/migrations. I don't think listing all migrations directories in flake8 exclude would be useful as an alternative, so unless anyone finds a solution, I think we can close this.

[Tim Graham]

Let's try this using autopep8 to manually format migration files in order to limit line length. I am getting tired of commenting about line length on pull requests, so automated enforcement seems worthwhile.

[Dražen Odobašić]

One way to deal with migrations is to exclude */0*.py which will not exclude django/db/migrations. The option also excludes some more esoteric migrations like ./tests/migrations/migrations_test_apps/conflicting_app_with_dependencies/migrations/0002_second.py and similar.

AFAICT, find -wholename '*/0*.py', no other files in Django repository, start with 0 and end with .py so it should be safe enough to use

Other than that, there are only 15 occurrences of E501 in migration files, 1018 problems remain :)

[Tim Graham]

I think it can be useful to run flake8 checks on migrations -- especially when writing them manually as in the case of data migrations. So I think fixing those 15 cases either manually or automatically would be ideal.

[Dražen Odobašić]

ok, so just to confirm we want to modify/refactor all lines that result with E501, in the Django master branch, right?

[Tim Graham]

Correct. I'd suggest to start with the instances in django/ to get a feel for any style decisions. Then you can proceed to tests/. Thanks!

[Dražen Odobašić]

Pull request: ​https://github.com/django/django/pull/5224

[Tim Graham <timograham@…>]

In b1e33ce:

Fixed #23395 -- Limited line lengths to 119 characters.