Experimental Bazel Python Rules

Status: This is ALPHA software.

Recent updates

• 2019-07-26: The canonical name of this repo has been changed from @io_bazel_rules_python to just @rules_python, in accordance with convention. Please update your WORKSPACE file and labels that reference this repo accordingly.

Rules

Core Python rules

• py_library
• py_binary
• py_test
• py_runtime
• py_runtime_pair

Packaging rules

• pip_import
• pip3_import

Overview

This repository provides two sets of Python rules for Bazel. The core rules provide the essential library, binary, test, and toolchain rules that are expected for any language supported in Bazel. The packaging rules provide support for integration with dependencies that, in a non-Bazel environment, would typically be managed by pip.

Historically, the core rules have been bundled with Bazel itself. The Bazel team is in the process of transitioning these rules to live in bazelbuild/rules_python instead. In the meantime, all users of Python rules in Bazel should migrate their builds to load these rules and their related symbols (PyInfo, etc.) from @rules_python instead of using built-ins or @bazel_tools//tools/python.

Setup

To use this repository, first modify your WORKSPACE file to load it and call the initialization functions as needed:

load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")

git_repository(
    name = "rules_python",
    remote = "https://github.com/bazelbuild/rules_python.git",
    # NOT VALID!  Replace this with a Git commit SHA.
    commit = "{HEAD}",
)

# This call should always be present.
load("@rules_python//python:repositories.bzl", "py_repositories")
py_repositories()

# This one is only needed if you're using the packaging rules.
load("@rules_python//python:pip.bzl", "pip_repositories")
pip_repositories()

Then in your BUILD files, load the core rules as needed with:

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

py_binary(
  name = "main",
  ...
)

Importing pip dependencies

These rules are designed to have developers continue using requirements.txt to express their dependencies in a Python idiomatic manner. These dependencies are imported into the Bazel dependency graph via a two-phased process in WORKSPACE:

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

# This rule translates the specified requirements.txt into
# @my_deps//:requirements.bzl, which itself exposes a pip_install method.
pip_import(
   name = "my_deps",
   requirements = "//path/to:requirements.txt",
)

# Load the pip_install symbol for my_deps, and create the dependencies'
# repositories.
load("@my_deps//:requirements.bzl", "pip_install")
pip_install()

The pip_import rule uses the system python command, which is usually Python 2. pip3_import uses the system python3 command.

Consuming pip dependencies

Once a set of dependencies has been imported via pip_import and pip_install we can start consuming them in our py_{binary,library,test} rules. In support of this, the generated requirements.bzl also contains a requirement method, which can be used directly in deps=[] to reference an imported py_library.

load("@my_deps//:requirements.bzl", "requirement")

py_library(
    name = "mylib",
    srcs = ["mylib.py"],
    deps = [
        ":myotherlib",
	# This takes the name as specified in requirements.txt
	requirement("importeddep"),
    ]
)

Canonical whl_library naming

It is notable that whl_library rules imported via pip_import are canonically named, following the pattern: pypi__{distribution}_{version}. Characters in these components that are illegal in Bazel label names (e.g. -, .) are replaced with _.

This canonical naming helps avoid redundant work to import the same library multiple times. It is expected that this naming will remain stable, so folks should be able to reliably depend directly on e.g. @pypi__futures_3_1_1//:pkg for dependencies, however, it is recommended that folks stick with the requirement pattern in case the need arises for us to make changes to this format in the future.

"Extras" will have a target of the extra name (in place of pkg above).

Development

Documentation

All of the content under docs/ besides the BUILD file is generated with Stardoc. To regenerate the documentation, simply run

./update_docs.sh

from the repository root.

Precompiled par files

The piptool.par and whltool.par files underneath tools/ are compiled versions of the Python scripts under the rules_python/ directory. We need to check in built artifacts because they are executed during WORKSPACE evaluation, before Bazel itself is able to build anything from source.

The .par files need to be regenerated whenever their sources are updated. This can be done by running

./update_tools.sh

from the repository root. However, since these files contain compiled code, we do not accept commits that modify them from untrusted sources. If you submit a pull request that modifies the sources and we accept the changes, we will regenerate these files for you before merging.