A macro is a function called from the BUILD file that can instantiate rules. Macros don’t give additional power, they are just used for encapsulation and code reuse. By the end of the loading phase, macros don’t exist anymore, and Bazel sees only the set of rules they created.
Native rules (i.e. rules that don’t need a
load() statement) can be
instantiated from the native module, e.g.
def my_macro(name, visibility=None): native.cc_library( name = name, srcs = ["main.cc"], visibility = visibility, )
If you need to know the package name (i.e. which BUILD file is calling the macro), use the function native.package_name().
bazel query --output=build //my/path:all will show you how the BUILD file
looks after evaluation. All macros, globs, loops are expanded. Known
select expressions are currently not shown in the output.
generator_function(which function generated the rules) or
generator_name(the name attribute of the macro), e.g.
$ bazel query --output=build 'attr(generator_function, my_macro, //my/path:all)'
To find out where exactly the rule
foo is generated in a BUILD file, you
can try the following trick. Insert this line near the top of the BUILD
cc_library(name = "foo"). Run Bazel. You will get an exception when
foo is created (due to a name conflict), which will show you the
full stack trace.
DEBUGlog line during the loading phase. Except in rare cases, either remove
debuggingparameter that defaults to
Falsebefore submitting the code to the depot.
If you want to throw an error, use the fail function. Explain clearly to the user what went wrong and how to fix their BUILD file. It is not possible to catch an error.
def my_macro(name, deps, visibility=None): if len(deps) < 2: fail("Expected at least two values in deps") # ...
All public functions (functions that don’t start with underscore) that
instantiate rules must have a
name argument. This argument should not be
optional (don’t give a default value).
Public functions should use a docstring following Python conventions.
In BUILD files, the
name argument of the macros must be a keyword argument
(not a positional argument).
name attribute of rules generated by a macro should include the name
argument as a prefix. For example,
macro(name = "foo") can generate a
foo and a genrule
In most cases, optional parameters should have a default value of
None can be passed directly to native rules, which treat it the same as if
you had not passed in any argument. Thus, there is no need to replace it
 for this purpose. Instead, the macro should defer
to the rules it creates, as their defaults may be complex or may change over
time. Additionally, a parameter that is explicitly set to its default value
looks different than one that is never set (or set to
None) when accessed
through the query language or build-system internals.
Macros should have an optional
The typical use-case for a macro is when you want to reuse a genrule, e.g.
genrule( name = "file", outs = ["file.txt"], cmd = "$(location generator) some_arg > $@", tools = [":generator"], )
If you want to generate another file with different arguments, you may want to extract this code to a function.
The BUILD file will become simply:
load("//path:generator.bzl", "file_generator") file_generator( name = "file", arg = "some_arg", )
In order to keep BUILD files clean and declarative, you must put the function in
.bzl file. For example, write the definition of the macro in
def file_generator(name, arg, visibility=None): native.genrule( name = name, outs = [name + ".txt"], cmd = "$(location generator) %s > $@" % arg, tools = ["//test:generator"], visibility = visibility, )
When you want to investigate what a macro does, use the following command to see the expanded form:
$ bazel query --output=build :file # /absolute/path/test/ext.bzl:42:3 genrule( name = "file", tools = ["//test:generator"], outs = ["//test:file.txt"], cmd = "$(location generator) some_arg > $@", )