Extensions are files with the
.bzl extension. Use the
load statement to
import a symbol from an extension.
This code will load the file
build_tools/rules/maprule.bzl and add the
maprule symbol to the environment. This can be used to load new rules,
functions or constants (e.g. a string, a list, etc.). Multiple symbols can be
imported by using additional arguments to the call to
load. Arguments must
be string literals (no variable) and
load statements must appear at
top-level, i.e. they cannot be in a function body.
load also supports aliases, i.e. you can assign different names to the
load("//build_tools/rules:maprule.bzl", maprule_alias = "maprule")
You can define multiple aliases within one
load statement. Moreover, the
argument list can contain both aliases and regular symbol names. The following
example is perfectly legal (please note when to use quotation marks).
load(":my_rules.bzl", "some_rule", nice_alias = "some_other_rule")
.bzl file, symbols starting with
_ are private and cannot be loaded
from another file. Visibility doesn't affect loading (yet): you don't need to
exports_files to make a
.bzl file visible.
A macro is a function that instantiates rules. It is useful when a
BUILD file is getting too repetitive or too complex, as it allows you to reuse
some code. The function is evaluated as soon as the BUILD file is read. After
the evaluation of the BUILD file, Bazel has little information about macros: if
your macro generates a
genrule, Bazel will behave as if you wrote the
genrule. As a result,
bazel query will only list the generated
A rule is more powerful than a macro. It can access Bazel internals and have full control over what is going on. It may for example pass information to other rules.
If you want to reuse simple logic, start with a macro. If a macro becomes complex, it is often a good idea to make it a rule. Support for a new language is typically done with a rule. Rules are for advanced users: we expect that most people will never have to write one, they will only load and call existing rules.
A build consists of three phases.
Loading phase. First, we load and evaluate all extensions and all BUILD files that are needed for the build. The execution of the BUILD files simply instantiates rules (each time a rule is called, it gets added to a graph). This is where macros are evaluated.
Analysis phase. The code of the rules is executed (their
function), and actions are instantiated. An action describes how to generate
a set of outputs from a set of inputs, e.g. "run gcc on hello.c and get
hello.o". It is important to note that we have to list explicitly which
files will be generated before executing the actual commands. In other words,
the analysis phase takes the graph generated by the loading phase and
generates an action graph.
Execution phase. Actions are executed, when at least one of their outputs is required. If a file is missing or if a command fails to generate one output, the build fails. Tests are also run during this phase.
Bazel uses parallelism to read, parse and evaluate the
.bzl files and
files. A file is read at most once per build and the result of the evaluation is
cached and reused. A file is evaluated only once all its dependencies (
statements) have been resolved. By design, loading a
.bzl file has no visible
side-effect, it only defines values and functions.
Bazel tries to be clever: it uses dependency analysis to know which files must be loaded, which rules must be analyzed, and which actions must be executed. For example, if a rule generates actions that we don't need for the current build, they will not be executed.
The following items are upcoming changes.
Comprehensions currently "leak" the values of their loop variables into the surrounding scope (Python 2 semantics). This will be changed so that comprehension variables are local (Python 3 semantics).
Previously dictionaries were guaranteed to use sorted order for their keys. Going forward, there is no guarantee on order besides that it is deterministic. As an implementation matter, some kinds of dictionaries may continue to use sorted order while others may use insertion order.
+= operator and similar operators are currently syntactic sugar;
y is the same as
x = x + y. This will change to follow Python semantics,
so that for mutable collection datatypes,
x += y will be a mutation to the
x rather than a rebinding of the variable
x itself to a new
value. E.g. for lists,
x += y will be the same as
The "set" datatype is being renamed to "depset" in order to avoid confusion with Python's sets, which behave very differently.
+ operator is defined for dictionaries, returning an immutable
concatenated dictionary created from the entries of the original
dictionaries. This will be going away. The same result can be achieved using
dict(a.items() + b.items()).
These changes concern the
load() syntax in particular.
load() statement can appear anywhere in a file so long as it is
at the top-level (not in an indented block of code). In the future they will
be required to appear at the beginning of the file, i.e., before any
In BUILD files,
load() can overwrite an existing variable with the loaded
symbol. This will be disallowed in order to improve consistency with .bzl
files. Use load aliases to avoid name clashes.
The .bzl file can be specified as either a path or a label. In the future only the label form will be allowed.
Cross-package visibility restrictions do not yet apply to loaded .bzl files.
At some point this will change. In order to load a .bzl from another package
it will need to be exported, such as by using an
The exact syntax has not yet been decided.
To profile your code and analyze the performance, use the
$ bazel build --nobuild --profile=/tmp/prof //path/to:target $ bazel analyze-profile /tmp/prof --html --html_details
Then, open the generated HTML file (
/tmp/prof.html in the example).