Extensions are files with the .bzl extension. Use the load statement to import a symbol from an extension.
load("//build_tools/rules:maprule.bzl", "maprule")
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 imported symbols.
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")
Symbols starting with _ are private and cannot be loaded from other files. Visibility doesn‘t affect loading (yet): you don’t need to use exports_files to make a .bzl file visible.
A macro is a function that instantiates rules. The function is evaluated as soon as the BUILD file is read. 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 genrule.
A rule is more powerful than a macro, as it can access Bazel internals and have full control over what is going on. It may for example pass information to other rules.
If a macro becomes complex, it is often a good idea to make it a rule.
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. This is where macros are evaluated.
Analysis phase. The code of the rules is executed (their implementation 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.
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 run during this phase, as they are actions.
Bazel uses parallelism to read, parse and evaluate the .bzl files and BUILD 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 (load() statements) have been resolved. By design, loading a .bzl file has no visible side-effect, it only defines values and functions.
The extension language (Skylark) is a superset of the Core Build Language and its syntax is a subset of Python. It is designed to be simple, thread-safe and integrated with the BUILD language. It is not a general-purpose language and most Python features are not included.
The following constructs have been added to the Core Build Language: if statements, for loops, and function definitions. They behave like in Python. Here is an example to show the syntax:
def fizz_buzz(n): """Print Fizz Buzz numbers from 1 to n.""" for i in range(1, n + 1): s = "" if i % 3 == 0: s += "Fizz" if i % 5 == 0: s += "Buzz" print(s if s else i) fizz_buzz(20)
Because evaluation of BUILD and .bzl files is performed in parallel, there are some restrictions in order to guarantee thread-safety and determinism. Two mutable data structures are available: lists and dicts. Unlike in Python, sets are not mutable.
In a build, there are many “evaluation contexts”: each .bzl file and each BUILD file is loaded in a different context. Each rule is also analyzed in a separate context. We allow side-effects (e.g. appending a value to a list or deleting an entry in a dictionary) only on objects created during the current evaluation context. Once the code in that context is done executing, all of its values are frozen.
For example, here is the content of the file foo.bzl:
var = [] def fct(): var.append(5) fct()
The variable var is created when foo.bzl is loaded. fct() is called during the same context, so it is safe. At the end of the evaluation, the environment contains an entry mapping the identifier var to a list [5]; this list is then frozen.
It is possible for multiple other files to load symbols from foo.bzl at the same time. For this reason, the following code is not legal:
load(":foo.bzl", "var", "fct") var.append(6) # runtime error, the list stored in var is frozen fct() # runtime error, fct() attempts to modify a frozen list
Evaluation contexts are also created for the analysis of each custom rule. This means that any values that are returned from the rule‘s analysis are frozen. Note that by the time a custom rule’s analysis begins, the .bzl file in which it is defined has already been loaded, and so the global variables are already frozen.
There are also restrictions on rebinding variables. In .bzl files, it is illegal to overwrite an existing global or built-in variable, such as by assigning to it, even when the module has not yet been frozen.
In addition to the mutability restrictions, there are also differences with Python:
All global variables cannot be reassigned.
for statements are not allowed at the top-level; factor them into functions instead.
Sets and dictionaries have a deterministic order of iteration (see documentation for sets).
Recursion is not allowed.
Sets have reference equality semantics and can be stored in other sets.
Lists and other mutable types may be stored in sets and in dictionary keys once they are frozen.
Modifying a collection during iteration is an error. You can avoid the error by iterating over a copy of the collection, e.g. for x in list(my_list): .... You can still modify its deep contents regardless.
Global (non-function) variables must be declared before they can be used in a function, even if the function is not called until after the global variable declaration. However, it is fine to define f() before g(), even if f() calls g().
The following Python features are not supported:
class (see struct function)import (see load statement)while, yield{2, 4, 6}) and set comprehensions ({2*x for x in [1, 2, 3]}). Instead, call set on lists (set([2, 4, 6])) and list comprehensions (set([2*x for x in [1, 2, 3]])).lambda and nested functionsis (use == instead)try, raise, except, finally (see fail for fatal errors).global, nonlocalThe 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.
The += operator and similar operators are currently syntactic sugar; x += 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 value of 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 x.extend(y).
The + 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()). Likewise, there is a + operator for sets that will be going away; users should use | instead.
The order comparison operators (<, <=, >=, >) are currently defined across different types of values, e.g., you can write 5 < 'foo'. This will be an error, just like in Python 3. Note that this means you will be unable to sort lists that contain mixed types of values.
The structure of the set that you get back from using the + or | operator is changing. Previously a + b, where a is a set, would include as its direct items all of a's direct items. Under the upcoming way, the result will only include a as a single transitive entity. This will alter the visible iteration order of the returned set. Most notably, set([1, 2]) + set([3, 4] + set([5, 6]) will return elements in the order 1 2 3 4 5 6 instead of 3 4 5 6 1 2. This change is associated with a fix that improves set union to be O(1) time.
The set datatype will be renamed in order to avoid confusion with Python's set datatype, which behaves very differently.
These changes concern the load() syntax in particular.
Currently a 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 non-load() statement.
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 exports_files declaration. The exact syntax has not yet been decided.