BUILD 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.

Contents

• Formatting example
• File structure
• References to targets in the current package
• Target naming
• Visibility
• Dependencies
• Globs
• Other conventions
• Differences with Python style guide

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 = True,
    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")

References to targets in the current package

Files should be referred to by their paths relative to the package directory (without ever using up-references, such as ..). Generated files should be prefixed with “:” to indicate that they are not sources. Source files should not be prefixed with :. Rules should be prefixed with :. For example, assuming x.cc is a source file:

cc_library(
    name = "lib",
    srcs = ["x.cc"],
    hdrs = [":gen-header"],
)

genrule(
    name = "gen-header",
    srcs = [],
    outs = ["x.h"],
    cmd = "echo 'int x();' > $@",
)

Target naming

Target names should be descriptive. If a target contains one source file, the target should generally be named after that source (e.g., a cc_library for chat.cc should be named “chat”).

The eponymous target for a package (the target with the same name as the containing directory) should provide the functionality described by the directory name. If there is no such target, do not create an eponymous target.

Prefer using the short name when referring to an eponymous target (//x instead of //x:x). If you are in the same package, prefer the local reference (:x instead of //x).

Visibility

Visibility should be scoped as tightly as possible, while still allowing access by tests and reverse dependencies. Use __pkg__ and __subpackages__ as appropriate.

Avoid setting package default_visibility to //visibility:public. //visibility:public should be individually set only for targets in the project’s public API. These could be libraries that are designed to be depended on by external projects or binaries that could be used by an external project’s build process.

Dependencies

Dependencies should be restricted to direct dependencies (dependencies needed by the sources listed in the rule). Do not list transitive dependencies.

Package-local dependencies should be listed first and referred to in a way compatible with the References to targets in the current package section above (not by their absolute package name).

Prefer to list dependencies directly, as a single list. Putting the “common” dependencies of several targets into a variable reduces maintainability, makes it impossible for tools to change the dependencies of a target and can lead to unused dependencies.

Globs

Indicate “no targets” with []. Do not use a glob that matches nothing: it is more error-prone and less obvious than an empty list.

Recursive

Do not use recursive globs (for example, glob(["**/*.java"])).

Recursive globs make BUILD files difficult to reason about because they skip subdirectories containing BUILD files.

Recursive globs are generally less efficient than having a BUILD file per directory with a dependency graph defined between them as this enables better forge caching and parallelism.

We recommend authoring a BUILD file per directory and defining a dependency graph between them instead.

Non-recursive

Non-recursive globs are generally acceptable.

Other 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 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.

• When setting boolean-type attributes, use boolean values, not integer values. For legacy reasons, rules will still convert integers to booleans as needed, but this is discouraged. Rationale: flaky = 1 could be misread as saying “deflake this target by rerunning it once”. flaky = True unambiguously says “this test is flaky”.

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.