Documentation

BUILD file style guide

In BUILD files, we take the same approach as in Go: We let the machine take care of most formatting issues. Buildifier is a tool that parses and emits the source code in a standard style. Every BUILD file is therefore formatted in the same automated way, which makes formatting a non-issue during code reviews. It also makes it easier for tools to understand, edit, and generate BUILD files.

BUILD file formatting must match the output of buildifier.

Formatting example

package(default_visibility = ["//visibility:public"])

py_test(
    name = "foo_test",
    srcs = glob(["*.py"]),
    data = [
        "//data/production/foo:startfoo",
        "//foo",
        "//third_party/java/jdk:jdk-k8",
    ],
    flaky = 1,
    deps = [
        ":check_bar_lib",
        ":foo_data_check",
        ":pick_foo_port",
        "//pyglib",
        "//testing/pybase",
    ],
)

File structure

We recommend to use the following order (every element is optional):

  • Package description (a comment)

  • All load() statements

  • The package() function.

  • Calls to rules and macros

Buildifier makes a distinction between a standalone comment and a comment attached to an element. If a comment is not attached to a specific element, use an empty line after it. The distinction is important when doing automated changes (e.g. to decide if we keep or remove a comment when we delete a rule).

# Standalone comment (e.g. to make a section in a file)

# Comment for the cc_library below
cc_library(name = "cc")

Conventions

  • Use uppercase and underscores to declare constants (e.g. GLOBAL_CONSTANT), use lowercase and underscores to declare variables (e.g. my_variable).

  • Labels should be canonicalized. Use //foo/bar instead of //foo/bar:bar. Use :bar if it is defined in the same package. Rationale: It makes clear if a label is local to a package. Sorting a list of labels is messy if all labels do not use the same conventions.

  • Labels should never be split, even if they are longer than 79 characters. Labels should be string literals whenever possible. Rationale: It makes find and replace easy. It also improves readability.

  • The value of the name attribute should be a literal constant string (except in macros). Rationale: External tools use the name attribute to refer a rule. They need to find rules without having to interpret code.

Differences with Python style guide

Although compatibility with Python style guide is a goal, there are a few differences:

  • No strict line length limit. Long comments and long strings are often split to 79 columns, but it is not required. It should not be enforced in code reviews or presubmit scripts. Rationale: Labels can be long and exceed this limit. It is common for BUILD files to be generated or edited by tools, which does not go well with a line length limit.

  • Implicit string concatenation is not supported. Use the + operator. Rationale: BUILD files contain many string lists. It is easy to forget a comma, which leads to a complete different result. This has created many bugs in the past. See also this discussion.

  • Use spaces around the = sign for keywords arguments in rules. Rationale: Named arguments are much more frequent than in Python and are always on a separate line. Spaces improve readability. This convention has been around for a long time, and we don't think it is worth modifying all existing BUILD files.

  • By default, use double quotation marks for strings. Rationale: This is not specified in the Python style guide, but it recommends consistency. So we decided to use only double-quoted strings. Many languages use double-quotes for string literals.

  • Use a single blank line between two top-level definitions. Rationale: The structure of a BUILD file is not like a typical Python file. It has only top-level statements. Using a single-blank line makes BUILD files shorter.