# Python Rules for Bazel

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## 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.

## 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/`](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.

See the [How to contribute](CONTRIBUTING.md) page for information on our
devlopment workflow.

## 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 just need to
[import the Federation](https://github.com/bazelbuild/bazel-federation#example-workspace)
and call the rules_python setup methods:

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

http_archive(
    name = "bazel_federation",
    url = "https://github.com/bazelbuild/bazel-federation/releases/download/0.0.1/bazel_federation-0.0.1.tar.gz",
    sha256 = "506dfbfd74ade486ac077113f48d16835fdf6e343e1d4741552b450cfc2efb53",
)

load("@bazel_federation//:repositories.bzl", "rules_python_deps")

rules_python_deps()
load("@bazel_federation//setup:rules_python.bzl",  "rules_python_setup")
rules_python_setup(use_pip=True)
```

Note the `use_pip` argument: rules_python may be imported either with or
without support for the packaging rules.

If you are not using the Federation, you can simply import rules_python
directly and call its initialization methods as follows:

```python
http_archive(
    name = "rules_python",
    url = "https://github.com/bazelbuild/rules_python/releases/download/0.0.1/rules_python-0.0.1.tar.gz",
    sha256 = "aa96a691d3a8177f3215b14b0edc9641787abaaa30363a080165d06ab65e1161",
)
load("@rules_python//python:repositories.bzl", "py_repositories")
py_repositories()
# Only needed if using the packaging rules.
load("@rules_python//python:pip.bzl", "pip_repositories")
pip_repositories()
```

To depend on a particular unreleased version (not recommended), you can
use `git_repository` instead of `http_archive`:

```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 with actual Git commit SHA.
    commit = "{HEAD}",
)

# Then load and call py_repositories() and possibly pip_repositories() as
# above.
```

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:

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

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

## Using the packaging rules

### Importing `pip` dependencies

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")

# 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()
```

### 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`.

```python
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"](
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.