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bazelbuild
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Description

Experimental Bazel Python Rules

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Python Rules for Bazel

  • Postsubmit Build status
  • Postsubmit + Current Bazel Incompatible Flags Build status

Recent updates

  • 2020-10-15: Release

    0.1.0
    was published, upstreaming the

    pip_install
    rule functionality from github.com/dillon-giacoppo/rulespythonexternal to address a number of long-standing issues with
    pip_import
    (eg. #96, #71, #102). Note that this is a backwards-incompatible release on account of the removal of
    pip_import
    from
    @rules_python//python:pip.bzl
    .
  • 2019-11-15: Added support for

    pip3_import
    (and more generally, a
    python_interpreter
    attribute to
    pip_import
    ). The canonical naming for wheel repositories has changed to accomodate loading wheels for both
    pip_import
    and
    pip3_import
    in the same build. To avoid breakage, please use
    requirement()
    instead of depending directly on wheel repo labels.
  • 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.

Overview

This repository is the home of the core Python rules --

py_library
,
py_binary
,
py_test
, and related symbols that provide the basis for Python support in Bazel. It also contains packaging rules for integrating with PyPI (
pip
). Documentation lives in the
docs/
directory and in the Bazel Build Encyclopedia.

Currently the core rules are bundled with Bazel itself, and the symbols in this repository are simple aliases. However, in the future the rules will be migrated to Starlark and debundled from Bazel. Therefore, the future-proof way to depend on Python rules is via this repository. See

Migrating from the Bundled Rules
below.

The core rules are stable. Their implementation in Bazel is subject to Bazel's backward compatibility policy. Once they are fully migrated to rules_python, they may evolve at a different rate, but this repository will still follow semantic versioning.

The packaging rules (

pip_install
, etc.) are less stable. We may make breaking changes as they evolve. There are no guarantees for rules underneath the
experimental/
directory.

This repository is maintained by the Bazel community. Neither Google, nor the Bazel team, provides support for the code. However, this repository is part of the test suite used to vet new Bazel releases. See the How to contribute page for information on our development workflow.

Getting started

To import rules_python in your project, you first need to add it to your

WORKSPACE
file:
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "rules_python",
    url = "https://github.com/bazelbuild/rules_python/releases/download/0.1.0/rules_python-0.1.0.tar.gz",
    sha256 = "b6d46438523a3ec0f3cead544190ee13223a52f6a6765a29eae7b7cc24cc83a0",
)

To depend on a particular unreleased version (not recommended), you can do:

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

rules_python_version = "c8c79aae9aa1b61d199ad03d5fe06338febd0774" # Latest @ 2020-10-15

http_archive( name = "rules_python", sha256 = "5be9610a959772697f57ec66bb58c8132970686ed7fb0f1cf81b22ddf12f5368", strip_prefix = "rules_python-{}".format(rules_python_version), url = "https://github.com/bazelbuild/rules_python/archive/{}.zip".format(rules_python_version), )

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

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

Using the packaging rules

The packaging rules create two kinds of repositories: A central repo that holds downloaded wheel files, and individual repos for each wheel's extracted contents. Users only need to interact with the central repo; the wheel repos are essentially an implementation detail. The central repo provides a

WORKSPACE
macro to create the wheel repos, as well as a function to call in
BUILD
files to translate a pip package name into the label of a
py_library
target in the appropriate wheel repo.

Importing
pip
dependencies

To add pip dependencies to your

WORKSPACE
load the
pip_install
function, and call it to create the individual wheel repos.
load("@rules_python//python:pip.bzl", "pip_install")

Create a central repo that knows about the dependencies needed for

requirements.txt.

pip_install( name = "my_deps", requirements = "//path/to:requirements.txt", )

Note that since pip is executed at WORKSPACE-evaluation time, Bazel has no information about the Python toolchain and cannot enforce that the interpreter used to invoke pip matches the interpreter used to run

py_binary
targets. By default,
pip_install
uses the system command
"python3"
. This can be overridden by passing the
python_interpreter
attribute or
python_interpreter_target
attribute to
pip_install
.

You can have multiple

pip_install
s in the same workspace, e.g. for Python 2 and Python 3. This will create multiple central repos that have no relation to one another, and may result in downloading the same wheels multiple times.

As with any repository rule, if you would like to ensure that

pip_install
is re-executed in order to pick up a non-hermetic change to your environment (e.g., updating your system
python
interpreter), you can completely flush out your repo cache with
bazel clean --expunge
.

Fetch
pip
dependencies lazily (experimental)

One pain point with

pip_install
is the need to download all dependencies resolved by your requirements.txt before the bazel analysis phase can start. For large python monorepos this can take a long time, especially on slow connections.

pip_parse
provides a solution to this problem. If you can provide a lock file of all your python dependencies
pip_parse
will translate each requirement into its own external repository. Bazel will only fetch/build wheels for the requirements in the subgraph of your build target.

There are API differences between

pip_parse
and
pip_install
: 1.
pip_parse
requires a fully resolved lock file of your python dependencies. You can generate this using
pip-compile
, or a virtualenv and
pip freeze
.
pip_parse
uses a label argument called
requirements_lock
instead of
requirements
to make this distinction clear. 2.
pip_parse
translates your requirements into a starlark macro called
install_deps
. You must call this macro in your WORKSPACE to declare your dependencies.
load("@rules_python//python:pip.bzl", "pip_parse")

Create a central repo that knows about the dependencies needed from

requirements_lock.txt.

pip_parse( name = "my_deps", requirements_lock = "//path/to:requirements_lock.txt", )

Load the starlark macro which will define your dependencies.

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

Call it to define repos for your requirements.

install_deps()

Importing
pip
dependencies with
pip_import
(legacy)

The deprecated

pip_import
can still be used if needed.
load("@rules_python//python/legacy_pip_import:pip.bzl", "pip_import", "pip_repositories")

Create a central repo that knows about the dependencies needed for requirements.txt.

pip_import( # or pip3_import name = "my_deps", requirements = "//path/to:requirements.txt", )

Load the central repo's install function from its //:requirements.bzl file, and call it.

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

An example can be found in

examples/legacy_pip_import
.

Consuming
pip
dependencies

Each extracted wheel repo contains a

py_library
target representing the wheel's contents. Rather than depend on this target's label directly -- which would require hardcoding the wheel repo's mangled name into your BUILD files -- you should instead use the
requirement()
function defined in the central repo's
//:requirements.bzl
file. This function maps a pip package name to a label.
load("@my_deps//:requirements.bzl", "requirement")

py_library( name = "mylib", srcs = ["mylib.py"], deps = [ ":myotherlib", requirement("some_pip_dep"), requirement("another_pip_dep"), ] )

For reference, the wheel repos are canonically named following the pattern:

@{central_repo_name}_pypi__{distribution}_{version}
. Characters in the distribution and version that are illegal in Bazel label names (e.g.
-
,
.
) are replaced with
_
. While this naming pattern doesn't change often, it is not guaranted to remain stable, so use of the
requirement()
function is recommended.

'Extras' requirement consumption

When using the legacy

pip_import
, you must specify the extra in the argument to the
requirement
macro. For example:
py_library(
    name = "mylib",
    srcs = ["mylib.py"],
    deps = [
        requirement("useful_dep[some_extra]"),
    ]
)

If using

pip_install
or
pip_parse
, any extras specified in the requirements file will be automatically linked as a dependency of the package so that you don't need to specify the extra. In the example above, you'd just put
requirement("useful_dep")
.

Consuming Wheel Dists Directly

If you need to depend on the wheel dists themselves, for instance to pass them
to some other packaging tool, you can get a handle to them with the

whl_requirement
macro. For example:
filegroup(  
    name = "whl_files", 
    data = [    
        whl_requirement("boto3"),   
    ]   
)

Migrating from the bundled rules

The core rules are currently available in Bazel as built-in symbols, but this form is deprecated. Instead, you should depend on rulespython in your

WORKSPACE
file and load the Python rules from `@rulespython//python:defs.bzl`.

A buildifier fix is available to automatically migrate

BUILD
and
.bzl
files to add the appropriate
load()
statements and rewrite uses of
native.py_*
.
# Also consider using the -r flag to modify an entire workspace.
buildifier --lint=fix --warnings=native-py 

Currently the

WORKSPACE
file needs to be updated manually as per Getting started above.

Note that Starlark-defined bundled symbols underneath

@bazel_tools//tools/python
are also deprecated. These are not yet rewritten by buildifier.

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