Getting started

The following two sections cover using rules_python with bzlmod and the older way of configuring bazel with a WORKSPACE file.

Using bzlmod

IMPORTANT: bzlmod support is still in Beta; APIs are subject to change.

The first step to using rules_python with bzlmod is to add the dependency to your MODULE.bazel file:

# Update the version "0.0.0" to the release found here:
# https://github.com/bazelbuild/rules_python/releases.
bazel_dep(name = "rules_python", version = "0.0.0")

Once added, you can load the rules and use them:

load("@rules_python//python:py_binary.bzl", "py_binary")

py_binary(...)

Depending on what you're doing, you likely want to do some additional configuration to control what Python version is used; read the following sections for how to do that.

Toolchain registration with bzlmod

A default toolchain is automatically configured depending on rules_python. Note, however, the version used tracks the most recent Python release and will change often.

If you want to use a specific Python version for your programs, then how to do so depends on if you‘re configuring the root module or not. The root module is special because it can set the default Python version, which is used by the version-unaware rules (e.g. //python:py_binary.bzl et al). For submodules, it’s recommended to use the version-aware rules to pin your programs to a specific Python version so they don't accidentally run with a different version configured by the root module.

Configuring and using the default Python version

To specify what the default Python version is, set is_default = True when calling python.toolchain(). This can only be done by the root module; it is silently ignored if a submodule does it. Similarly, using the version-unaware rules (which always use the default Python version) should only be done by the root module. If submodules use them, then they may run with a different Python version than they expect.

python = use_extension("@rules_python//python/extensions:python.bzl", "python")

python.toolchain(
    python_version = "3.11",
    is_default = True,
)

Then use the base rules from e.g. //python:py_binary.bzl.

Pinning to a Python version

Pinning to a version allows targets to force that a specific Python version is used, even if the root module configures a different version as a default. This is most useful for two cases:

  1. For submodules to ensure they run with the appropriate Python version
  2. To allow incremental, per-target, upgrading to newer Python versions, typically in a mono-repo situation.

To configure a submodule with the version-aware rules, request the particular version you need, then use the @python_versions repo to use the rules that force specific versions:

python = use_extension("@rules_python//python/extensions:python.bzl", "python")

python.toolchain(
    python_version = "3.11",
)
use_repo(python, "python_versions")

Then use e.g. load("@python_versions//3.11:defs.bzl", "py_binary") to use the rules that force that particular version. Multiple versions can be specified and use within a single build.

For more documentation, see the bzlmod examples under the {gh-path}examples folder. Look for the examples that contain a MODULE.bazel file.

Other toolchain details

The python.toolchain() call makes its contents available under a repo named python_X_Y, where X and Y are the major and minor versions. For example, python.toolchain(python_version="3.11") creates the repo @python_3_11. Remember to call use_repo() to make repos visible to your module: use_repo(python, "python_3_11")

Using a WORKSPACE file

To import rules_python in your project, you first need to add it to your WORKSPACE file, using the snippet provided in the release you choose

To depend on a particular unreleased version, you can do the following:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")


# Update the SHA and VERSION to the lastest version available here:
# https://github.com/bazelbuild/rules_python/releases.

SHA="84aec9e21cc56fbc7f1335035a71c850d1b9b5cc6ff497306f84cced9a769841"

VERSION="0.23.1"

http_archive(
    name = "rules_python",
    sha256 = SHA,
    strip_prefix = "rules_python-{}".format(VERSION),
    url = "https://github.com/bazelbuild/rules_python/releases/download/{}/rules_python-{}.tar.gz".format(VERSION,VERSION),
)

load("@rules_python//python:repositories.bzl", "py_repositories")

py_repositories()

Toolchain registration

To register a hermetic Python toolchain rather than rely on a system-installed interpreter for runtime execution, you can add to the WORKSPACE file:

load("@rules_python//python:repositories.bzl", "python_register_toolchains")

python_register_toolchains(
    name = "python_3_11",
    # Available versions are listed in @rules_python//python:versions.bzl.
    # We recommend using the same version your team is already standardized on.
    python_version = "3.11",
)

load("@python_3_11//:defs.bzl", "interpreter")

load("@rules_python//python:pip.bzl", "pip_parse")

pip_parse(
    ...
    python_interpreter_target = interpreter,
    ...
)

After registration, your Python targets will use the toolchain's interpreter during execution, but a system-installed interpreter is still used to ‘bootstrap’ Python targets (see https://github.com/bazelbuild/rules_python/issues/691). You may also find some quirks while using this toolchain. Please refer to python-build-standalone documentation's Quirks section.

Toolchain usage in other rules

Python toolchains can be utilized in other bazel rules, such as genrule(), by adding the toolchains=["@rules_python//python:current_py_toolchain"] attribute. You can obtain the path to the Python interpreter using the $(PYTHON2) and $(PYTHON3) “Make” Variables. See the {gh-path}test_current_py_toolchain <tests/load_from_macro/BUILD.bazel> target for an example.

“Hello World”

Once you've imported the rule set into your WORKSPACE using any of these methods, you can then load the core rules in your BUILD files with the following:

load("@rules_python//python:defs.bzl", "py_binary")

py_binary(
  name = "main",
  srcs = ["main.py"],
)