Code Style Guide

This section defines a style guide which should be followed by all code that is written in Drake. Being consistent with this style will make the code easier to read, debug, and maintain. To ensure your code is style compliant, consider using tools for complying with coding style.

See also the brief Code Review Checklist, where a list of the most frequent problems are collected.

Note: Many of the files in the repository were written before this style guide, or did not follow it precisely. If you find style errors, go ahead and change it and submit a pull request.

C++ Style

The Drake C++ style guide (which is derived from the Google C++ style guide) can be found on its own page. Its source lives in the styleguide repository.

Python Style

Drake strictly follows PEP 8 – Style Guide for Python Code except for the specific clarifications, exceptions, and additional rules noted below. Since PEP 8 incorporates PEP 257 – Docstring Conventions, Drake follows its recommendations as well.

Drake also follows the RobotLocomotion fork of the Google Python Style Guide. Please refer to that page for the sections that are adhered to, and the relevant exceptions.

See tools for complying with coding style for details about the automated style checks.

Clarifications

  • External, third-party, and auto-generated source files are not to be checked for style.
  • Always prefer long, human-readable variable/method/class names to short acronyms.

Exceptions

  • Lines containing a long URL may be longer than 80 columns if necessary to avoid splitting the URL.

Additional Rules

  • When importing in-tree modules, always use absolute import paths; explicit relative import paths are disallowed. See the PEP 8 discussion of imports for more detail.

  • Sometimes __init__.py files are necessary, for Python’s import mechanism; these files should be non-empty (via a copyright notice, for example). Rationale: 0-byte files can be mistakenly perceived as the result of some error or accident.

  • When using the logging module, avoid its lazy-formatting syntax. Rationale: exceptions raised in lazy formatting get printed to stderr, but are otherwise ignored, and thus may escape notice.

  • Executable Python files should be limited to only scripts which are not run via Bazel-generated Python proxy scripts [1]. If a script qualifies, use the following “shebang” line:

    #!/usr/bin/env python3
    

    Rationale: /usr/bin/env enables a PATH search for the Python 3 executable. This is also recommended by PEP 394.

[1]Generally, this means scripts that run via bazel run, bazel test, or ./bazel-bin/....

Java Style

We also strictly follow the Google Java Style Guide. Here are some additional comments:

  • Every class and method should have a brief _javadoc_ associated with it.
  • All Java classes should be in packages relative to the Drake root,
    e.g.: package drake.examples.Pendulum

LCM Style

  • LCM types are under_scored with a leading lcmt_ added. If the type is specific to a particular robot, then it begins with lcmt_robotname_.
  • Variable names in LCM types follow the rules above.

package.xml Style

Robot Operating System (ROS) organizes code and data into packages. Each package is located in its own directory, which contains a file called package.xml. The official instructions on what this file should contain is given here. Drake uses this same file for defining and finding packages. Specifically, it is searched for by populatePackageMap(). Note that package.xml files are necessary even if you’re not using Drake with ROS because the model files used by Drake (e.g., URDF and SDF), frequently refer to resources like mesh files via a package:// syntax, whose full paths are resolved by the package.xml files.

When adding a model to Drake (typically in drake/examples/), you will need to add a package.xml file to the example’s directory to enable modeling files like URDF and SDF to refer to resources like mesh files contained within the example’s directory. Please ensure that your package.xml file contains every required field. The following minimal package.xml file can get you started:

<!--
This XML file is used by:
  drake/systems/plants/xmlUtil.cpp
Method:
  searchDirectory()
-->

<package format="2">
  <name>package_name</name>
  <version>0.0.0</version>
  <description>
    A description of your package.
  </description>
  <maintainer email="drake-users@mit.edu">Drake Users</maintainer>
  <license>BSD</license>
</package>

In the above example, replace “package_name” with the name of your package. This is typically the name of the directory holding the package.xml file.

Version numbers

We’ll adopt the following convention for version numbers in Drake: The version number will have the format W.X.Y.Z where

  • W = major release number
  • X = minor release number
  • Y = development stage*
  • Z = build

Development stage is one of four values:

  • 0 = alpha (buggy, not for use)
  • 1 = beta (mostly bug-free, needs more testing)
  • 2 = release candidate (rc) (stable)
  • 3 = release

Z (build) is optional. This is probably not needed but could just refer to the revision of the repository at the time of snapshot. Numbered versions should be referenced via tags.