# Chromium Python Style Guide

_For other languages, please see the [Chromium style
guides](https://chromium.googlesource.com/chromium/src/+/main/styleguide/styleguide.md)._

As of 2021-05-12, Chromium is transitioning from Python 2 to Python 3 (follow
[crbug.com/941669](https://crbug.com/941669) for updates). See
[//docs/python3_migration.md](../docs/python3_migration.md) for more on
how to migrate code.

For new (Python 3) code, you can assume Python 3.8 (and that's what the bots
will use), but we are increasingly seeing people running 3.9 locally as well.

We (often) use a tool called [vpython] to manage Python packages; vpython
is a wrapper around virtualenv. However, it is not safe to use vpython
regardless of context, as it can have performance issues. All tests are
run under vpython, so it is safe there, but you should not use vpython
during PRESUBMIT checks, gclient runhooks, or during the build unless
a [//build/OWNER](../../build/OWNERS) has told you that it is okay to do so.

Also, there is some performance overhead to using vpython, so prefer not
to use vpython unless you need it (to pick up packages not available in the
source tree).

"shebang lines" (the first line of many unix scripts, like `#!/bin/sh`)
aren't as useful as you might think in Chromium, because
most of our python invocations come from other tools like Ninja or
the swarming infrastructure, and they also don't work on Windows.
So, don't expect them to help you.

However, if your script is executable, you should still use one, and for
Python you should use `#!/usr/bin/env/python3` or `#!/usr/bin/env vpython3`
in order to pick up the right version of Python from your $PATH rather than
assuming you want the version in `/usr/bin`; this allows you to pick up the
versions we endorse from
`depot_tools`.

Chromium follows [PEP-8](https://www.python.org/dev/peps/pep-0008/).

It is also encouraged to follow advice from
[Google's Python Style Guide](https://google.github.io/styleguide/pyguide.html),
which is a superset of PEP-8.

See also:
* [Chromium OS Python Style Guide](https://sites.google.com/a/chromium.org/dev/chromium-os/python-style-guidelines)
* [Blink Python Style Guide](blink-python.md)

[TOC]

## Our Previous Python Style

Chromium used to differ from PEP-8 in the following ways:
* Use two-space indentation instead of four-space indentation.
* Use `CamelCase()` method and function names instead of `unix_hacker_style()`
  names.
* 80 character line limits rather than 79.

New scripts should not follow these deviations, but they should be followed when
making changes to files that follow them.

## Making Style Guide Changes

You can propose changes to this style guide by sending an email to
`python@chromium.org`. Ideally, the list will arrive at some consensus and you
can request review for a change to this file. If there's no consensus,
[`//styleguide/python/OWNERS`](https://chromium.googlesource.com/chromium/src/+/main/styleguide/python/OWNERS)
get to decide.

## Tools

### pylint
[Depot tools](http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools.html)
contains a local copy of pylint, appropriately configured.
* Directories need to opt into pylint presubmit checks via:
   `input_api.canned_checks.RunPylint()`.

### YAPF
[YAPF](https://github.com/google/yapf) is the Python formatter used by:

```sh
git cl format --python
```

Directories can opt into enforcing auto-formatting by adding a `.style.yapf`
file with the following contents:
```
[style]
based_on_style = pep8
```

Entire files can be formatted (rather than just touched lines) via:
```sh
git cl format --python --full
```

YAPF has gotchas. You should review its changes before submitting. Notably:
 * It does not re-wrap comments.
 * It won't insert characters in order wrap lines. You might need to add ()s
   yourself in order to have to wrap long lines for you.
 * It formats lists differently depending on whether or not they end with a
   trailing comma.


#### Bugs
* Are tracked here: https://github.com/google/yapf/issues.
* For Chromium-specific bugs, please discuss on `python@chromium.org`.

#### Editor Integration
See: https://github.com/google/yapf/tree/main/plugins

[vpython]: https://chromium.googlesource.com/infra/infra/+/refs/heads/main/doc/users/vpython.md