What is Starlark?

Starlark

Starlark is a configuration language designed by Google, originally for Bazel. It features Python-like syntax with deterministic evaluation, making it ideal for build systems and configuration.

Key Features

Deterministic

Same inputs always produce same outputs. No randomness, no timestamps.

Hermetic

No filesystem, network, or system access. Safe for untrusted code.

Parallel

Modules load concurrently. Immutable data ensures thread safety.

Python-like

Familiar syntax reduces learning curve for Python developers.

Quick Example

# Define data
people = {
    "Alice": 22,
    "Bob": 40,
    "Charlie": 14,
}

# Define a function
def greet(name):
    """Return a greeting."""
    return "Hello, {}!".format(name)

# List comprehension
adults = [name for name, age in people.items() if age >= 18]

print(greet(", ".join(adults)))  # Hello, Alice, Bob!

Why Starlark?

Use Case	Benefit
Build systems	Reproducible builds via determinism
Configuration	Type-safe, programmable configs
Sandboxed scripting	Safe execution of user code
CI/CD pipelines	Consistent behavior across runs

Who Uses Starlark?

• Bazel - Google’s build system
• Buck2 - Meta’s build system
• Tilt - Kubernetes development
• Drone CI - Continuous integration
• Kurtosis - Development environments
• ytt - YAML templating

Differences from Python

While Starlark looks like Python, key differences ensure determinism:

Feature	Python	Starlark
Mutation	Allowed	Frozen after creation
Global state	Yes	No
Recursion	Yes	No (prevents infinite loops)
while loops	Yes	No (use for with ranges)
import	Yes	Use load() instead
Classes	Yes	No (use structs)

Next Steps

Language Basics Learn Starlark syntax and features

Resources Official docs, tools, and community