README.md - third_party/github/bazelbuild/rules_python - Git at Google

 # Python Rules for Bazel

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 ## Recent updates

 * 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](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
 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/0.0.1/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

 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

 Adding pip dependencies to your `WORKSPACE` is a two-step process. First you
 declare the central repository using `pip_import`, which invokes pip to read
 a `requirements.txt` file and download the appropriate wheels. Then you load
 the `pip_install` function from the central repo, and call it to create the
 individual wheel repos.

 **Important:** If you are using Python 3, load and call `pip3_import` instead.

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

 # 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()
 ```

 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_import` uses the system command `"python"`, which on most
 platforms is a Python 2 interpreter. This can be overridden by passing the
 `python_interpreter` attribute to `pip_import`. `pip3_import` just acts as a
 wrapper that sets `python_interpreter` to `"python3"`.

 You can have multiple `pip_import`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_import` is
 reexecuted 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`.

 ### 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. (["Extras"](
 https://packaging.python.org/tutorials/installing-packages/#installing-setuptools-extras)
 can be referenced using the `pkg[extra]` syntax.)

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

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

 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.

 ## 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.
	# Python Rules for Bazel

	* Postsubmit [![Build status](https://badge.buildkite.com/0bcfe58b6f5741aacb09b12485969ba7a1205955a45b53e854.svg?branch=master)](https://buildkite.com/bazel/python-rules-python-postsubmit)
	* Postsubmit + Current Bazel Incompatible Flags [![Build status](https://badge.buildkite.com/219007166ab6a7798b22758e7ae3f3223001398ffb56a5ad2a.svg?branch=master)](https://buildkite.com/bazel/rules-python-plus-bazelisk-migrate)

	## Recent updates

	* 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](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
	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/0.0.1/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

	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

	Adding pip dependencies to your `WORKSPACE` is a two-step process. First you
	declare the central repository using `pip_import`, which invokes pip to read
	a `requirements.txt` file and download the appropriate wheels. Then you load
	the `pip_install` function from the central repo, and call it to create the
	individual wheel repos.

	Important: If you are using Python 3, load and call `pip3_import` instead.

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

	# 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()
	```

	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_import` uses the system command `"python"`, which on most
	platforms is a Python 2 interpreter. This can be overridden by passing the
	`python_interpreter` attribute to `pip_import`. `pip3_import` just acts as a
	wrapper that sets `python_interpreter` to `"python3"`.

	You can have multiple `pip_import`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_import` is
	reexecuted 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`.

	### 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. (["Extras"](
	https://packaging.python.org/tutorials/installing-packages/#installing-setuptools-extras)
	can be referenced using the `pkg[extra]` syntax.)

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

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

	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.

	## 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.