You should follow this guide for all Python source code that is created as part of the Chromium OS project i.e. src/platform/*. You may not like some bits of it, but you still need to follow it. In doing so, you ensure that others can work with your code.
This guide may be ignored if you are working on code that is part of an open source project that already conforms to a different style i.e. generally Python code in src/third_party. Autotest is a great example of this - see the other notable Python style guides below.
See also the overall Chromium coding style.
The Google Python Style guide is the official Python style guide for Chromium OS original code.
New projects should use that unmodified public Python Google style guide (4 space indent with methods_with_underscores). Historically, we adopted a style that was congruent with Google internal Python style guide (2 spaces with MethodsAsCamelCase). Individual older projects are in the process of migrating to the public style as time allows; there is no mandate to migrate.
That is, for new projects, use:
If that guide is silent on an issue, then you should fall back to PEP-8.
PEP-257 is referenced by PEP-8. If it isn't covered in the Comments section, you can also fall back to PEP-257.
There are a number of other Python style guidelines out there.
PEP-257 says that "The docstring for a function or method should summarize its behavior and document its arguments, return value(s), side effects, exceptions raised, and restrictions. However, it doesn't specify any details of what this should look like.
Fortunately, the Google Python style guide provides a reasonable syntax for arguments and return values. We will use that as our standard.
The content of each subsection (e.g. Args, Returns, etc...) should use the same indentation level as the file. If the file is using 2 spaces, then the subsection content should use 2 space indents (as seen in the example below). If the file is using 4 space indents, then the subsection content should use 4 space indents as well. In both situations, use 4 space hanging indentation for long argument descriptions (as seen with keys: below).
All docstring content should use full sentences (proper capitalization & periods), including argument descriptions (as seen below). This applies even when the docstring is a one-liner.
def FetchBigtableRows(big_table, keys, other_silly_variable=None): """Fetches rows from a Bigtable. Retrieves rows pertaining to the given keys from the Table instance represented by big_table. Silly things may happen if other_silly_variable is not None. Args: big_table: An open Bigtable Table instance. keys: A sequence of strings representing the key of each table row to fetch. other_silly_variable: Another optional variable, that has a much longer name than the other args, and which does nothing. Returns: A dict mapping keys to the corresponding table row data fetched. Each row is represented as a tuple of strings. For example: {'Serak': ('Rigel VII', 'Preparer'), 'Zim': ('Irk', 'Invader'), 'Lrrr': ('Omicron Persei 8', 'Emperor')} If a key from the keys argument is missing from the dictionary, then that row was not found in the table. Raises: IOError: An error occurred accessing the bigtable.Table object. """ pass
Specifically:
Args: section, using the format above.Examples:, Args:, Returns: (or Yields: with generators), and Raises: should be in that order and come last in the docstring, each separated by a blank line.Examples: and Attributes: are permitted. When the the __init__ method arguments line up with the class‘s attributes (e.g., the function does a lot of self.foo = foo), there is no need to duplicate argument documentation in the __init__ docstring. Putting information in the class’s attributes is preferred.The following rules apply to imports (in addition to rules already talked about in PEP-8):
from __future__ import absolute_import syntax should be used (see PEP-328).import *. Always be explicit about what you are importing. Using import * is useful for interactive Python but shouldn't be used in checked-in code.All files that are meant to be executed should start with the line:
#!/usr/bin/env python2
If your system doesn't support that, it is sufficiently unusual for us to not worry about compatibility with it. Most likely it will break in other fun ways.
Files that are not meant to be executed should not contain a shebang line.
It is acceptable to mix f-strings & non-f-string formatting in a codebase, module, function, or any other grouping. Do not worry about having to pick only one style.
When writing Python 3.6+ code, prefer f-strings. They aren't always the best fit which is why we say you should prefer rather than always use them. See the next section for non-f-string formatting.
Writing readable f-strings is important, so try to avoid packing too much logic in the middle of the {...} sections. If you need a list comprehension or complicated computation, then it's usually better to pull it out into a temporary variable instead. Dynamic code in the middle of static strings can be easy to miss for readers.
The Google style guide says that the % formatting and '{}'.format() styles are both permitted. In CrOS we exclusively use % formatting, so you should too to match. There is no real material difference between the two styles, so sticking with % for consistency is better.
Changing either form to an f-string is always OK.
x = 'name: %s; score: %d' % (name, n) x = 'name: %(name)s; score: %(score)d' % {'name': name, 'score': n}
Keep in mind that for logging type functions, we don't format in-place. Instead, we pass them as args to the function.
logging.info('name: %s; score: %d', name, n)
When using comprehensions & generators, it's best if you stick to “throw away” variable names. “x” tends to be the most common iterator.
some_var = [x for x in some_list if x]
While helping with readability (since these expressions are supposed to be kept “simple”), it also helps avoid the problem of variable scopes not being as tight as people expect.
# This throws an exception because the variable is undefined. print(x) some_var = [x for x in (1, 2)] # This displays "1" because |x| is now defined. print(x) string = 'some string' some_var = [string for string in ('a', 'b', 'c')] # This displays "c" because the earlier |string| value has been clobbered. print(string)
It is OK to leave TODOs in code. Why? Encouraging people to at least document parts of code that need to be thought out more (or that are confusing) is better than leaving this code undocumented.
See also the Google style guide here.
All TODOs should be formatted like:
TODO(username): Revisit this code when the frob feature is done.
...where username is your chromium.org username. If you don't have a chromium.org username, please use an email address.
Please do not use other forms of TODO (like TBD, FIXME, XXX, etc). This makes TODOs much harder for someone to find in the code.
Python code written for Chromium OS should be “pylint clean”.
Pylint is a utility that analyzes Python code to look for bugs and for style violations. We run it in Chromium OS because:
Pylint is notoriously picky and sometimes warns about things that are really OK. Because of this, we:
You can use cros lint so that we can be sure everyone is on the same version. This should work outside and inside the chroot.
All Python modules should have a corresponding unit test module called <original name>_unittest.py in the same directory. We use a few related modules (they're available in the chroot as well):
mock (import mock) for testing mocks
import mox) is the old mock frameworkYou'll still see code using this as not all have been migrated to mock yet
unittest (import unittest) for unit test suites and methods for testing
All main modules require a main method. Use this boilerplate at the end:
if __name__ == '__main__': sys.exit(main(sys.argv[1:]))
main accept the arguments instead of going through the implicit global sys.argv state allows for better reuse & unittesting: other modules are able to import your module and then invoke it with main(['foo', 'bar']).sys.exit allows main to return values which makes it easy to unittest, and easier to reason about behavior in general: you can always use return and not worry if you‘re in a func that expects a sys.exit call. If your func doesn’t explicitly return, then you still get the default behavior of sys.exit(0).sys.argv[0] at all as the caller can set that to whatever it wants (it is never guaranteed to be the basename of your script). You should set a constant in your module, or you should base it off the filename.# A constant string that is guaranteed to never change. _NAME = 'foo' # The filename which should be stable (respects symlink names). _NAME = os.path.basename(__file__) # The filename which derefs symlinks. _NAME = os.path.basename(os.path.realpath(__file__))
argv[0], you should use sys.argv[0] directly. If you're trying to print an error message, then usually you want to use argparse (see below) and the parser.error() helper instead.Main modules should have a symlink without an extension that point to the .py file. All actual code should be in .py files.
GFlags or optparse anymore -- stick to argparse everywhere.