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DEP 0008: Formatting Code with Black

DEP:0008
Author: Aymeric Augustin
Implementation Team:Aymeric Augustin, Carlton Gibson, Florian Apolloner, Herman Schistad, Markus Holtermann
Shepherd:Andrew Godwin
Status: Draft
Type:Process
Created:2019-04-27
Last-Modified:2019-05-10

Abstract

This DEP proposes to enforce code formatting with Black in Django.

Foreword

Code formatting is the ultimate bikeshed. Perfectionists have strong feelings about the aesthetics of laying out the tokens that make a Python program in the window of a text editor.

Within the principles of PEP 8, there's quite a bit of room for formatting code in various ways. Even these principles aren't fully consensual, notably the rules about hanging indents — that's why flake8 disables controversial rules by default.

The django-developers thread about Black was the largest by number of messages in several months. Any proposal in this area will have enthusiastic proponents and sincere opponents. Consensus is an elusive goal when personal taste is such a large factor.

I'm writing this DEP because, based on my experience with other open-source projects and the community feedback I received — in public or in private — I believe that adopting Black would be a worthwhile improvement for Django.

Keep in mind that this isn't a make-or-break decision for Django. It's about making it more convenient to contribute, especially for new or occasional contributors who haven't internalized current code formatting standards.

Like most changes to entrenched habits, it's a trade-off between current contributors and future contributors, where current contributors get to make the decision.

Even though this DEP argues in favor of Black, I made an effort to represent both sides of the debate fairly, with relevant quotes from the django-developers discussion.

Please read it with an open mind :-)

Specification

All Python code in Django is formatted with Black, using its default settings, that is, 88 characters per line and double quotes.

Official Django projects are encouraged to adopt the same policy. Some already do, for example channels and asgiref.

Motivation

There's no debate about the usefulness of readable code formatting. While it doesn't change the semantics of the code, it simplifies future maintenance. Since code is read much more often than it is written, it makes sense to invest some effort when writing code to make it easier to read later.

Until now this effort has been performed by humans, mostly based on PEP 8, a rather long document. Memorizing and applying properly all these rules isn't the most exciting part of contributing to Django, to say the least.

Furthermore, code formatting can create a barrier for new contributors, if they think they can't write code that looks as good as pull requests from other contributors. This is particularly true for new contributors, who are increasingly hard to attract as Django grows more mature.

With automated code formatting, this workload can be transferred to computers, saving human bandwidth for higher value-added activities.

Generally speaking, there are two major benefits to automated code formatting:

  • Individually, developers no longer have to constantly make small decisions about code formatting, based on rules, on taste, or on a mix of both. Just throw the code in a file, run the formatter, done!

    You don't realize how much this reduces decision fatigue until you try it. It's a real boon to those, like me, who spend way too much time fine-tuning the formatting of their code.

  • Collectively, contributors no longer need to think about code formatting in code reviews. Whatever the formatter does is the expected result, by definition.

    This removes the thought stream of "mmm, I would have formatted this a bit differently" while reviewing code exactly like it removes "mmm, how do I format this?" while writing code.

Or, to put it more concisely: "Not needing to think much about code style when writing code or when reviewing is very nice." - Ian Foote

Specifically for Django, I would add a third major benefit:

  • Would-be contributors have one fewer hurdle to jump to create a merge-ready pull request. Even committers with a decade of experience writing code for Django say the back and forth of style reviews can be annoying!

    Django has over a thousand open accepted tickets, many of which aren't very hard and just need focused effort on a small area. (Look at the admindocs tickets for example.) At the same time, many people express a desire to get involved and yet don't.

    The stringent review process, while critical for quality, is a barrier to entry. It seems silly that formatting would put people off, but it does. Making it a non-issue would help.

In the words of a contributor: "One of the main frustration points I've had when making a contribution is having to fix small formatting errors. (...) It produces a lot of inertia and can stop PRs from getting merged for extended periods of time. So from a new contributor angle I think Black is an obvious choice." - Nick Sarbicki

Finally, automated code formatting will increase consistency across the Django code base. Currently the style of each module shows roughly when it was written or rewritten. Having a unified style will reduce the friction of adjusting to the style of each module. This is a nice side effect.

Given these benefits, if formatting code was an entirely mechanical process, it would have been automated long ago! Unfortunately, it's hard to design an algorithm that works sufficiently well for all practical cases.

Black is the first Python code formatter that produces good enough results and demonstrates significant traction in open source projects.

Therefore, this DEP proposes to adopt Black for Django.

Rationale

If you're reading this, I assume that you familiarized yourself with Black and that I don't need to explain its philosophy.

The discussion of adopting Black for Django revolves around two topics:

  1. Process: there's consensus that automated code formatting would greatly facilitate the development of Django;
  2. Results: there's no consensus at the time of writing: some people like what Black produces, others don't.

Process

There's consensus in favor of automated code formatting, even if different people assign different weight to each reason laid out in the "Motivation" section above.

Even those who oppose Black or express skepticism recognize the advantages. Here are relevant quotes from the django-developers discussion:

  • "I'm not sure I like Black per se, but using an auto-formatter would enable review comments to focus on substantive points." - Carlton Gibson
  • "I like the idea of an autoformatter. I dislike the particular mostly-unconfigurable style Black enforces, and I find that several of its rules negatively impact code readability." - James Bennett
  • "I see the benefits [lower barrier to entry, time saving for the Fellows, etc], but I don't believe Black is the answer." - Curtis Maloney

So this point is well established.

Results

Opinions are mixed regarding the quality of what Black produces.

Obviously Black gives consistent results faster than humans. This may not seem useful to contributors with enough Python experience to format code pretty much like Black would without much effort. It's more valuable to contributors who haven't reached that stage yet. It levels the coding field.

Humans are very sensitive to cases where a computer does worse than humans, even if the computer does better on average. The occasional obviously wrong result has a devastating effect on the acceptability of automation. This is the most common argument brought against Black.

(And it is in no way specific to Black. All automation efforts in the history of humanity must have received similar criticism at some point.)

Several developers report that, in their experience, Black made code formatting worse and decreased readability. Concrete examples shown in the discussion were short lists, which Black reformats when they fit on a single line, and vertically aligned comments, which Black is unable to preserve. This is being addressed in Black and is expected to be resolved before Black becomes stable.

These issues don't seem critical enough to rule out Black. As a last resort, Black could be disabled locally with # fmt: off and # fmt: on if it gets the formatting of a block of code egregiously wrong.

Others explained that, after an initial knee-jerk reaction against change, they started to like Black's choices. For example:

  • "As for disagreeing with some of Black's choices - you learn very quickly to live with those choices, and forget those choices. (...) I'm in favour of using Black's double quotes for strings. I hated this decision when it was first made, but have seriously come around to it, and prefer it aesthetically too." - Josh Smeaton
  • "I've used Black extensively on several projects, and much like f-strings, the last Pink Floyd album, and broccoli, have found I really like something I didn't think I would." - Tim Allen
  • "I'm one of those people who hesitated because I didn't like many of the choices Black made but I adapted really quickly." - Matthias Kestenholz

Looking at how Black reformats a few files from the Django source tree, I'm impressed by how few changes it makes. What Black produces is very close to the current Django coding style. That should make its adoption painless.

The obvious exception is quotes. Black standardizes on double quotes while Django uses single and double quotes inconsistently. Some parts of Django — including parts I wrote more recently — use single quotes for machine-readable identifiers and double quotes for human-readable language. In hindsight, this convention is too hard to enforce to be worth the effort, all the more since it isn't generalized. Going forwards, normalizing to double quotes like Black does by default will keep things simple.

My best guess is that Black will make code formatting a bit better on average, despite occasional sub-optimal results. Others may feel more strongly about their personal preferences that diverge from what Black does. However, I don't think personal preferences should outweigh growing community standards.

Ultimately, given how much this is a matter of personal judgement, perhaps the best attempt at consensus would be to state that the formatting produced by Black doesn't make a decisive change, positive or negative, to our ability to maintain Django.

Timeline

At this time, Black is in beta. The formatting it produces could still change. This DEP will be implemented once Black is stable.

Other concerns

Reformatting the entire code base with Black will touch most files and change many lines without altering their meaning. This will pollute the git history. However, formatting changes already happen alongside new features and bug fixes, adding a steady stream of pollution. Adopting Black will eliminate future code reformatting, making the git history cleaner looking forwards.

Besides, GitHub has a "View blame prior to this change" button to jump easily through refactoring commits. git-hyper-blame was also suggested as a way to ignore non-significant commits, although it requires some setup and doesn't integrate with IDEs.

Reformatting will also be disruptive for open pull requests. One way to update them is to run Black on modified files, keep a copy aside, start a new branch from master, move the modified files back into place, and commit the result.

In order to minimize the effort for backporting patches, Black will be applied to the master and stable/2.2.x branches, which are in their mainstream support period. 2.2 is an LTS release that will be supported for three more years; this is a good reason for formatting it. Black will not be applied to stable/2.1.x and stable/1.11.x which are in the extended support period and only get fixes for security and data loss bugs.

Alternatives

Three major Python code formatters exist: autopep8, yapf and Black.

No one argued in favor autopep8. Also I believe Black's approach is superior.

It was suggested that yapf could be configured to produce results closer to Django's current style. I don't think that's worth pursuing for three reasons:

  • The point of adopting an automatic code formatter isn't to have our own Django-flavored code formatting style. It's about making our Python code look as much as possible like what everyone else in the Python community writes. Code formatters maximize their usefulness by not being configurable.
  • Reaching consensus in open source communities is hard — I'm investing more than a day in writing this DEP! — which makes it essential to minimize choices. This must be why non-configurable formatters such as Prettier and Black have seen fast adoption by open source projects.
  • Black produces formatting that is so close to Django's current standards that there seems to be very little value in tuning a yapf configuration to produce something even closer.

It was also suggested to build a tool that could only reformat statements modified by a given commit. This would resolve the concerns about polluting the git history while preserving many benefits of this proposal, even if making non-standard tool part of the workflow isn't ideal. Unfortunately, no such tool exists and no one is volunteering to build it.

Finally, as Black is maintained in the Python GitHub organization, it has a good chance of becoming a standard tool of the Python community.

Summary

To sum up:

  1. Applying Black to the source code of Django won't make formatting drastically better or worse;
  2. Integrating Black in the development process of Django has very significant benefits;
  3. These benefits clearly outweigh code formatting style considerations.

One final quote: "The best thing about automatic formatters, in my opinion, is even if you don't like the style at least you don't have to talk about it anymore! And you tend to get used to it eventually." - Sam Cooke

Backwards Compatibility

This DEP doesn't introduce any backwards incompatibilities.

Black guarantees that it doesn't change the behavior of the code by checking that processing a file doesn't change its AST.

Reference Implementation

Implementing this change requires:

  1. Updating the coding style:
    • Adding documentation about Black, similar to the existing documentation about isort. The doc will make it clear that the # fmt: off escape hatch is allowed only in extreme cases where Black produces unreadable code, not whenever someone disagrees with the style choices of Black.
    • Updating explanations around PEP 8 and flake8 — they cover a lot more than code formatting so they remain useful even with Black.
    • Removing other references to code formatting, like the specification of the favorite hanging indent style and chained calls style.
  1. Updating flake8 and isort configuration to be compatible with Black.

    This is straightforward and well documented.

  2. Formatting the code. This will be done in three steps for each branch:

    • Identify if Black produces an egregiously bad result on some files. For example, the date and time formats files were noted as possible problems. Exclude these files with # fmt: off and # fmt: on comments.

    • Run Black on the entire Django code repository and make a single commit, which will look like this: https://github.com/hermansc/django/pull/1.

      Since the change will be fully automated, there won't be anything to review, so it's easier to make just one commit. That commit will be easy to identify as non-significant in the future.

    • Attempt to refactor excluded files, perhaps by moving comments, so that Black can do a decent job on them. Commit this separately. This isn't strictly necessary. It can be done at any later point.

  3. Enforcing Black in CI. This means:

    • Adding a black builder to Jenkins, based on the isort builder;
    • Adding a black job to tox.ini.

Copyright

This document has been placed in the public domain per the Creative Commons CC0 1.0 Universal license (http://creativecommons.org/publicdomain/zero/1.0/deed).

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