Starlark Language

This page is an overview of Starlark, formerly known as Skylark, the language used in Bazel. For a complete list of functions and types, see the Bazel API reference.

For more information about the language, see Starlark's GitHub repo.

For the authoritative specification of the Starlark syntax and behavior, see the Starlark Language Specification.


Starlark's syntax is inspired by Python3. This is valid syntax in Starlark:

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)


Starlark's semantics can differ from Python, but behavioral differences are rare, except for cases where Starlark raises an error. The following Python types are supported:


Starlark favors immutability. Two mutable data structures are available: lists and dicts. Changes to mutable data-structures, such as appending a value to a list or deleting an entry in a dictionary are valid only for objects created in the current context. After a context finishes, its values become immutable.

This is because Bazel builds use parallel execution. During a build, each .bzl file and each BUILD file get their own execution context. Each rule is also analyzed in its own context.

Let's go through an example with the file foo.bzl:

# `foo.bzl`
var = [] # declare a list

def fct(): # declare a function
  var.append(5) # append a value to the list

fct() # execute the fct function

Bazel creates var when foo.bzl loads. var is thus part of foo.bzl's context. When fct() runs, it does so within the context of foo.bzl. After evaluation for foo.bzl completes, the environment contains an immutable entry, var, with the value [5].

When another bar.bzl loads symbols from foo.bzl, loaded values remain immutable. For this reason, the following code in bar.bzl is illegal:

# `bar.bzl`
load(":foo.bzl", "var", "fct") # loads `var`, and `fct` from `./foo.bzl`

var.append(6)  # runtime error, the list stored in var is frozen

fct()          # runtime error, fct() attempts to modify a frozen list

Global variables defined in bzl files cannot be changed outside of the bzl file that defined them. Just like the above example using bzl files, values returned by rules are immutable.

Differences between BUILD and .bzl files

BUILD files register targets via making calls to rules. .bzl files provide definitions for constants, rules, macros, and functions.

Native functions and native rules are global symbols in BUILD files. bzl files need to load them using the native module.

There are two syntactic restrictions in BUILD files: 1) declaring functions is illegal, and 2) *args and **kwargs arguments are not allowed.

Differences with Python

  • Global variables are immutable.

  • for statements are not allowed at the top-level. Use them within functions instead. In BUILD files, you may use list comprehensions.

  • if statements are not allowed at the top-level. However, if expressions can be used: first = data[0] if len(data) > 0 else None.

  • Deterministic order for iterating through Dictionaries.

  • Recursion is not allowed.

  • Int type is limited to 32-bit signed integers. Overflows will throw an error.

  • Modifying a collection during iteration is an error.

  • Except for equality tests, comparison operators <, <=, >=, >, etc. are not defined across value types. In short: 5 < 'foo' will throw an error and 5 == "5" will return false.

  • In tuples, a trailing comma is valid only when the tuple is between parentheses — when you write (1,) instead of 1,.

  • Dictionary literals cannot have duplicated keys. For example, this is an error: {"a": 4, "b": 7, "a": 1}.

  • Strings are represented with double-quotes (such as when you call repr).

  • Strings aren't iterable.

The following Python features are not supported:

  • implicit string concatenation (use explicit + operator).
  • Chained comparisons (such as 1 < x < 5).
  • class (see struct function).
  • import (see load statement).
  • while, yield.
  • float and set types.
  • generators and generator expressions.
  • is (use == instead).
  • try, raise, except, finally (see fail for fatal errors).
  • global, nonlocal.
  • most builtin functions, most methods.