Skylark is a language that defines how software is built, and as such it is both a programming and a configuration language.
You will use Skylark to write BUILD files, macros, and build rules. Macros and rules are essentially meta-languages - they define how BUILD files are written. BUILD files are intended to be simple and repetitive.
All software is read more often than it is written. This is especially true for Skylark, as engineers read BUILD files to understand dependencies of their targets and details of their builds.This reading will often happen in passing, in a hurry, or in parallel to accomplishing some other task. Consequently, simplicity and readability are very important so that users can parse and comprehend BUILD files quickly.
When a user opens a BUILD file, they quickly want to know the list of targets in the file; or review the list of sources of that C++ library; or remove a dependency from that Java binary. Each time you add a layer of abstraction, you make it harder for a user to do these tasks.
BUILD files are also analyzed and updated by many different tools. Tools may not be able to edit your BUILD file if it uses abstractions. Keeping your BUILD files simple will allow you to get better tooling. As a code base grows, it becomes more and more frequent to do changes across many BUILD files in order to update a library or do a cleanup.
Do not create a macro just to avoid some amount of repetition in BUILD files. The DRY principle doesn’t really apply here. The goal is not to make the file shorter; the goal is to make your files easy to process, both by humans and tools.
When in doubt, follow the Python style guide. In particular, use 4 4 spaces for indentation (we previously recommended 2, but we now follow the Python convention).
Document files and functions using docstrings. Use a docstring at the top of each .bzl file, and a docstring for each public function.
Rules and aspects, along with their attributes, as well as providers and their fields, should be documented using the doc argument.
Variables and function names use lowercase with words separated by underscores ([a-z][a-z0-9_]*), e.g. cc_library. Top-level private values start with one underscore. Bazel enforces that private values cannot be used from other files. Local variables should not use the underscore prefix.
As in BUILD files, there is no strict line length limit as labels can be long. When possible, try to use at most 79 characters per line.
In keyword arguments, spaces around the equal sign are optional, but be consistent within any given call. In general, we follow the BUILD file convention when calling macros and native rules, and the Python convention for other functions, e.g.
def fct(name, srcs): filtered_srcs = my_filter(source = srcs) native.cc_library( name = name, srcs = filtered_srcs, testonly = True, )
Prefer values True and False instead of 0 and 1 for boolean values (e.g. when using a boolean attribute in a rule).
Do not use the print() function in production code; it is only intended for debugging, and will spam all direct and indirect users of your .bzl file. The only exception is that you may submit code that uses print() if it is disabled by default and can only be enabled by editing the source -- for example, if all uses of print() are guarded by if DEBUG: where DEBUG is hardcoded to false. Be mindful of whether these statements are useful enough to justify their impact on readability.
A macro is a function which instantiates one or more rules during the loading phase. In general, use rules whenever possible instead of macros. The build graph seen by the user is not the same as the one used by Bazel during the build - macros are expanded before Bazel does any build graph analysis.
Because of this, when something goes wrong, the user will need to understand your macro’s implementation to troubleshoot build problems. Additionally, bazel query results can be hard to interpret because targets shown in the results come from macro expansion. Finally, aspects are not aware of macros, so tooling depending on aspects (IDEs and others) might fail.
A safe use for macros is leaf nodes, such as macros defining test permutations: in that case, only the “end users” of those targets need to know about those additional nodes, and any build problems introduced by macros are never far from their usage.
For macros that define non-leaf nodes, follow these best practices:
name argument and define a target with that name. That target becomes that macro's main target._, include the name attribute as a prefix, and have restricted visibility.deps, name as you would the attribute (see below).Engineers often write macros when the Skylark API of relevant rules is insufficient for their specific use case, regardless of whether the rule is defined within Bazel in native code, or in Skylark. If you’re facing this problem, ask the rule author if they can extend the API to accomplish your goals.
As a rule of thumb, the more macros resemble the rules, the better.
py_extension. For most languages, typical rules include:*_library - a compilation unit or “module”.*_binary - a target producing an executable or a deployment unit.*_test - a test target. This can include multiple tests. Expect all tests in a *_test target to be variations on the same theme, for example, testing a single library.*_import: a target encapsulating a pre-compiled artifact, such as a .jar, or a .dll that is used during compilation.srcs: label_list, allowing files: source files, typically human-authored.deps: label_list, typically not allowing files: compilation dependencies.data: label_list, allowing files: data files, such as test data etc.runtime_deps: label_list: runtime dependencies that are not needed for compilation.doc keyword argument to the attribute's declaration (attr.label_list() or similar).myrule the name _myrule_impl.