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

 # Python Rules for Bazel

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 ## 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/main/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_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](CONTRIBUTING.md) 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, using the snippet provided in the
 [release you choose](https://github.com/bazelbuild/rules_python/releases)

 To depend on a particular unreleased version, you can do:

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

 rules_python_version = "740825b7f74930c62f44af95c9a4c1bd428d2c53" # Latest @ 2021-06-23

 http_archive(
     name = "rules_python",
     sha256 = "3474c5815da4cb003ff22811a36a11894927eda1c2e64bf2dac63e914bfdf30f",
     strip_prefix = "rules_python-{}".format(rules_python_version),
     url = "https://github.com/bazelbuild/rules_python/archive/{}.zip".format(rules_python_version),
 )
 ```

 To register a hermetic Python toolchain rather than rely on whatever is already on the machine, you can add to the `WORKSPACE` file:

 ```python
 load("@rules_python//python:repositories.bzl", "python_register_toolchains")

 python_register_toolchains(
     name = "python3_9",
     # Available versions are listed in @rules_python//python:versions.bzl.
     # We recommend using the same version your team is already standardized on.
     python_version = "3.9",
 )

 load("@python3_9//:defs.bzl", "interpreter")

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

 pip_parse(
     ...
     python_interpreter_target = interpreter,
     ...
 )
 ```

 > You may find some quirks while using this toolchain.
 > Please refer to [this link](https://python-build-standalone.readthedocs.io/en/latest/quirks.html) for details.

 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

 Usage of the packaging rules involves two main steps.

 1. [Installing `pip` dependencies](#installing-pip-dependencies)
 2. [Consuming `pip` dependencies](#consuming-pip-dependencies)

 The packaging rules create two kinds of repositories: A central external repo that holds
 downloaded wheel files, and individual external repos for each wheel's extracted
 contents. Users only need to interact with the central external repo; the wheel repos
 are essentially an implementation detail. The central external repo provides a
 `WORKSPACE` macro to create the wheel repos, as well as a function, `requirement()`, for use in
 `BUILD` files that translates a pip package name into the label of a `py_library`
 target in the appropriate wheel repo.

 ### Installing `pip` dependencies

 To add pip dependencies to your `WORKSPACE`, load the `pip_install` function, and call it to create the
 central external repo and individual wheel external repos.


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

 # Create a central external repo, @my_deps, that contains Bazel targets for all the
 # third-party packages specified in the requirements.txt file.
 pip_install(
    name = "my_deps",
    requirements = "//path/to:requirements.txt",
 )
 ```

 Note that since `pip_install` is a repository rule and therefore executes pip 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. This will create multiple external 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 force it to re-execute by running
 `bazel sync --only [pip_install name]`.

 ### Fetch `pip` dependencies lazily

 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 by using the `compile_pip_requirements` rule,
    running `pip-compile` directly, or using 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.


 ```python
 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()
 ```

 ### Consuming `pip` dependencies

 Each extracted wheel repo contains a `py_library` target representing
 the wheel's contents. There are two ways to access this library. The
 first is using the `requirement()` function defined in the central
 repo's `//:requirements.bzl` file. This function maps a pip package
 name to a label:

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

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

 The reason `requirement()` exists is that the pattern for the labels,
 while not expected to change frequently, is not guaranteed to be
 stable. Using `requirement()` ensures that you do not have to refactor
 your `BUILD` files if the pattern changes.

 On the other hand, using `requirement()` has several drawbacks; see
 [this issue][requirements-drawbacks] for an enumeration. If you don't
 want to use `requirement()` then you can instead use the library
 labels directly. For `pip_parse` the labels are of the form

 ```
 @{name}_{package}//:pkg
 ```

 Here `name` is the `name` attribute that was passed to `pip_parse` and
 `package` is the pip package name with characters that are illegal in
 Bazel label names (e.g. `-`, `.`) replaced with `_`. If you need to
 update `name` from "old" to "new", then you can run the following
 buildozer command:

 ```
 buildozer 'substitute deps @old_([^/]+)//:pkg @new_${1}//:pkg' //...:*
 ```

 For `pip_install` the labels are instead of the form

 ```
 @{name}//pypi__{package}
 ```

 [requirements-drawbacks]: https://github.com/bazelbuild/rules_python/issues/414

 #### 'Extras' requirement consumption

 When using the legacy `pip_import`, you must specify the extra in the argument to the `requirement` macro. For example:

 ```python
 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:

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

	## 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/main/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_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](CONTRIBUTING.md) 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, using the snippet provided in the
	[release you choose](https://github.com/bazelbuild/rules_python/releases)

	To depend on a particular unreleased version, you can do:

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

	rules_python_version = "740825b7f74930c62f44af95c9a4c1bd428d2c53" # Latest @ 2021-06-23

	http_archive(
	name = "rules_python",
	sha256 = "3474c5815da4cb003ff22811a36a11894927eda1c2e64bf2dac63e914bfdf30f",
	strip_prefix = "rules_python-{}".format(rules_python_version),
	url = "https://github.com/bazelbuild/rules_python/archive/{}.zip".format(rules_python_version),
	)
	```

	To register a hermetic Python toolchain rather than rely on whatever is already on the machine, you can add to the `WORKSPACE` file:

	```python
	load("@rules_python//python:repositories.bzl", "python_register_toolchains")

	python_register_toolchains(
	name = "python3_9",
	# Available versions are listed in @rules_python//python:versions.bzl.
	# We recommend using the same version your team is already standardized on.
	python_version = "3.9",
	)

	load("@python3_9//:defs.bzl", "interpreter")

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

	pip_parse(
	...
	python_interpreter_target = interpreter,
	...
	)
	```

	> You may find some quirks while using this toolchain.
	> Please refer to [this link](https://python-build-standalone.readthedocs.io/en/latest/quirks.html) for details.

	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

	Usage of the packaging rules involves two main steps.

	1. [Installing `pip` dependencies](#installing-pip-dependencies)
	2. [Consuming `pip` dependencies](#consuming-pip-dependencies)

	The packaging rules create two kinds of repositories: A central external repo that holds
	downloaded wheel files, and individual external repos for each wheel's extracted
	contents. Users only need to interact with the central external repo; the wheel repos
	are essentially an implementation detail. The central external repo provides a
	`WORKSPACE` macro to create the wheel repos, as well as a function, `requirement()`, for use in
	`BUILD` files that translates a pip package name into the label of a `py_library`
	target in the appropriate wheel repo.

	### Installing `pip` dependencies

	To add pip dependencies to your `WORKSPACE`, load the `pip_install` function, and call it to create the
	central external repo and individual wheel external repos.


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

	# Create a central external repo, @my_deps, that contains Bazel targets for all the
	# third-party packages specified in the requirements.txt file.
	pip_install(
	name = "my_deps",
	requirements = "//path/to:requirements.txt",
	)
	```

	Note that since `pip_install` is a repository rule and therefore executes pip 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. This will create multiple external 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 force it to re-execute by running
	`bazel sync --only [pip_install name]`.

	### Fetch `pip` dependencies lazily

	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 by using the `compile_pip_requirements` rule,
	running `pip-compile` directly, or using 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.


	```python
	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()
	```

	### Consuming `pip` dependencies

	Each extracted wheel repo contains a `py_library` target representing
	the wheel's contents. There are two ways to access this library. The
	first is using the `requirement()` function defined in the central
	repo's `//:requirements.bzl` file. This function maps a pip package
	name to a label:

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

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

	The reason `requirement()` exists is that the pattern for the labels,
	while not expected to change frequently, is not guaranteed to be
	stable. Using `requirement()` ensures that you do not have to refactor
	your `BUILD` files if the pattern changes.

	On the other hand, using `requirement()` has several drawbacks; see
	[this issue][requirements-drawbacks] for an enumeration. If you don't
	want to use `requirement()` then you can instead use the library
	labels directly. For `pip_parse` the labels are of the form

	```
	@{name}_{package}//:pkg
	```

	Here `name` is the `name` attribute that was passed to `pip_parse` and
	`package` is the pip package name with characters that are illegal in
	Bazel label names (e.g. `-`, `.`) replaced with `_`. If you need to
	update `name` from "old" to "new", then you can run the following
	buildozer command:

	```
	buildozer 'substitute deps @old_([^/]+)//:pkg @new_${1}//:pkg' //...:*
	```

	For `pip_install` the labels are instead of the form

	```
	@{name}//pypi__{package}
	```

	[requirements-drawbacks]: https://github.com/bazelbuild/rules_python/issues/414

	#### 'Extras' requirement consumption

	When using the legacy `pip_import`, you must specify the extra in the argument to the `requirement` macro. For example:

	```python
	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:

	```python
	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 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.