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:
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)
Skylark is syntactically a subset of both Python 2 and Python 3, and will remain so through at least the 1.x release lifecycle. This ensures that Python-based tooling can at least parse Skylark code. Although Skylark is not semantically a subset of Python, behavioral differences are rare (excluding cases where Skylark raises an error).
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.
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
var =  def fct(): var.append(5) fct()
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
; this list is
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.
In addition to the mutability restrictions, there are also differences with Python:
Global variables cannot be reassigned.
for statements are not allowed at the top-level; factor them into functions
Dictionaries have a deterministic order of iteration.
Recursion is not allowed.
Int type is limited to 32-bit signed integers.
Lists and other mutable types may be stored 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
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
g(), even if
The order comparison operators (<, <=, >=, >) are not defined across different
types of values, e.g., you can't compare
5 < 'foo' (however you still can
compare them using == or !=). This is a difference with Python 2, but
consistent with Python 3. Note that this means you are unable to sort lists
that contain mixed types of values.
Tuple syntax is more restrictive. You may use a trailing comma only when the
tuple is between parentheses, e.g. write
(1,) instead of
Strings are represented with double-quotes (e.g. when you call repr).
The following Python features are not supported:
lambdaand nested functions
failfor fatal errors)