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Expand the Overview. Add more context on core rules vs packaging rules and
level of support. Add instructions for using Bazel Federation. Move pip stuff
to sub-headings.
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@brandjon
brandjon authored Oct 9, 2019
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# Experimental Bazel Python Rules

Status: This is **ALPHA** software.
# Python Rules for Bazel

[![Build status](https://badge.buildkite.com/0bcfe58b6f5741aacb09b12485969ba7a1205955a45b53e854.svg)](https://buildkite.com/bazel/python-rules-python-postsubmit)

## 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](https://docs.bazel.build/versions/master/skylark/deploying.html#workspace). Please update your WORKSPACE file and labels that reference this repo accordingly.

## Rules

### Core Python rules

* [py_library](docs/python.md#py_library)
* [py_binary](docs/python.md#py_binary)
* [py_test](docs/python.md#py_test)
* [py_runtime](docs/python.md#py_runtime)
* [py_runtime_pair](docs/python.md#py_runtime_pair)

### Packaging rules

* [pip_import](docs/pip.md#pip_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`.
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/`](https://github.com/bazelbuild/rules_python/tree/master/docs)
directory and in the
[Bazel Build Encyclopedia](https://docs.bazel.build/versions/master/be/python.html).

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`](#Migrating-from-the-bundled-rules) below.

The core rules are stable. Their implementation in Bazel is subject to Bazel's
[backward compatibility policy](https://docs.bazel.build/versions/master/backward-compatibility.html).
Once they are fully migrated to rules_python, they may evolve at a different
rate, but this repository will still follow
[semantic versioning](https://semver.org).

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

## Getting started

To import rules_python in your project, you first need to add it to your
`WORKSPACE` file. If you are using the [Bazel
Federation](https://github.com/bazelbuild/bazel-federation), you will want to
copy the boilerplate in the rules_python release's notes, under the "WORKSPACE
setup" heading. This will look something like the following:

## Setup
```python
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
name = "rules_python",
# NOT VALID: Replace with actual version and SHA.
url = "https://github.com/bazelbuild/rules_python/releases/download/<RELEASE>/rules_python-<RELEASE>.tar.gz",
sha256 = "<SHA>",
)
load("@rules_python//python:repositories.bzl", "py_repositories", "rules_python_toolchains")
py_repositories()
rules_python_toolchains()
```

To use this repository, first modify your `WORKSPACE` file to load it and call
the initialization functions as needed:
Otherwise, you may import rules_python in a standalone way by copying the
following:

```python
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.
# NOT VALID: Replace with actual 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:
Either way, you can then load the core rules in your `BUILD` files with:

``` python
load("@rules_python//python:defs.bzl", "py_binary")
Expand All @@ -72,12 +82,14 @@ py_binary(
)
```

## Importing `pip` dependencies
## Using the packaging rules

### 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`:
The packaging 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`:

```python
load("@rules_python//python:pip.bzl", "pip_import")
Expand All @@ -95,7 +107,7 @@ load("@my_deps//:requirements.bzl", "pip_install")
pip_install()
```

## Consuming `pip` dependencies
### 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
Expand All @@ -116,7 +128,7 @@ py_library(
)
```

## Canonical `whl_library` naming
### 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
Expand All @@ -134,34 +146,57 @@ format in the future.
https://packaging.python.org/tutorials/installing-packages/#installing-setuptools-extras)
will have a target of the extra name (in place of `pkg` above).

## 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 rules_python in your
WORKSPACE file and load the Python rules from `@rules_python//python:defs.bzl`.

A [buildifier](https://github.com/bazelbuild/buildtools/blob/master/buildifier/README.md)
fix is available to automatically migrate BUILD and .bzl files to add the
appropriate `load()` statements and rewrite uses of `native.py_*`.

```sh
# Also consider using the -r flag to modify an entire workspace.
buildifier --lint=fix --warnings=native-py <files>
```

Currently the WORKSPACE file needs to be updated manually as per [Getting
started](#Getting-started) above.

Note that Starlark-defined bundled symbols underneath
`@bazel_tools//tools/python` are also deprecated. These are not yet rewritten
by buildifier.

## Development

### Documentation

All of the content under `docs/` besides the `BUILD` file is generated with
Stardoc. To regenerate the documentation, simply run
The content underneath `docs/` is generated. To update the documentation,
simply run this in the root of the repository:

```shell
./update_docs.sh
```

from the repository root.

### Precompiled par files
### 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
versions of the Python scripts under the `packaging/` 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

```shell
# You can pass --nodocker if Docker is not available on your system.
./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.
we do not accept commits that modify them from untrusted sources.<sup>1</sup>
If you submit a pull request that modifies the sources and we accept the
changes, we will regenerate these files for you before merging.

<sup>1</sup> See "[Reflections on Trusting Trust](https://en.wikipedia.org/wiki/Backdoor_(computing)#Compiler_backdoors)".

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