styleguide/python/python.md - chromium/src - Git at Google

 # Chromium Python Style Guide

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

 The minimum supported Python version is 3.9, we recommend using Python 3.11 (as
 in, that's what the bots use, so don't assume or require anything older *or*
 newer), but most newer versions of Python3 will work fine for most things. There
 is an appropriate version of Python3 in `$depot_tools/python-bin`, if you don't
 have one already.

 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, and vpython is the default for running scripts during
 PRESUBMIT checks (input_api.python3_executable points to vpython3 and is used in
 GetPythonUnitTests), but you should not use vpython during 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. That said, a python 3
 shebang is one way to indicate to the presubmit system that test scripts should
 be run under Python 3 rather than Python 2.

 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:

 - [ChromiumOS Python Style Guide](https://chromium.googlesource.com/chromiumos/docs/+/HEAD/styleguide/python.md)
 - [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.

 ## Portability

 There are a couple of differences in how text files are handled on Windows that
 can lead to portability problems. These differences are:

 - The default encoding when reading/writing text files is cp1252 on Windows and
   utf-8 on Linux, which can lead to Windows-only test failures. These can be
   avoided by always specifying `encoding='utf-8'` when opening text files.

 - The default behavior when writing text files on Windows is to emit \\r\\n
   (carriage return line feed) line endings. This can lead to cryptic
   Windows-only test failures and is generally undesirable. This can be avoided
   by always specifying `newline=''` when opening text files for writing.

 That is, use these forms when opening text files in Python:

 - reading: with open(filename, 'r', encoding='utf-8') as f:
 - writing: with open(filename, 'w', encoding='utf-8', newline='') as f:

 ## 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
 [`python@chromium.org`]: https://groups.google.com/a/chromium.org/g/python
	# Chromium Python Style Guide

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

	The minimum supported Python version is 3.9, we recommend using Python 3.11 (as
	in, that's what the bots use, so don't assume or require anything older or
	newer), but most newer versions of Python3 will work fine for most things. There
	is an appropriate version of Python3 in `$depot_tools/python-bin`, if you don't
	have one already.

	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, and vpython is the default for running scripts during
	PRESUBMIT checks (input_api.python3_executable points to vpython3 and is used in
	GetPythonUnitTests), but you should not use vpython during 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. That said, a python 3
	shebang is one way to indicate to the presubmit system that test scripts should
	be run under Python 3 rather than Python 2.

	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:

	- [ChromiumOS Python Style Guide](https://chromium.googlesource.com/chromiumos/docs/+/HEAD/styleguide/python.md)
	- [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.

	## Portability

	There are a couple of differences in how text files are handled on Windows that
	can lead to portability problems. These differences are:

	- The default encoding when reading/writing text files is cp1252 on Windows and
	utf-8 on Linux, which can lead to Windows-only test failures. These can be
	avoided by always specifying `encoding='utf-8'` when opening text files.

	- The default behavior when writing text files on Windows is to emit \\r\\n
	(carriage return line feed) line endings. This can lead to cryptic
	Windows-only test failures and is generally undesirable. This can be avoided
	by always specifying `newline=''` when opening text files for writing.

	That is, use these forms when opening text files in Python:

	- reading: with open(filename, 'r', encoding='utf-8') as f:
	- writing: with open(filename, 'w', encoding='utf-8', newline='') as f:

	## 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
	[`python@chromium.org`]: https://groups.google.com/a/chromium.org/g/python