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docs(gazelle): Migrate Gazelle docs to ReadTheDocs, part 2/5: installation and usage (#3132)
Part of #3082 2nd of probably 5 PRs. + Migrate installation and usage info from `gazelle/README.md` to `gazelle/docs/installation_and_usage.md` + Slight rewording and reformatting of the `Example` section. + Reorganized and modernized the `MODULE.bazel` and `BUILD.bazel` examples: + less details on rules_python as that's available in other docs + default to gazelle multilang support now that #3057 is merged. + Mechanical updates: + Wrap at ~80 chars + Use MyST directives and roles.
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‎gazelle/README.md‎

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:::
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## Example
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We have an example of using Gazelle with Python located [here](https://github.com/bazel-contrib/rules_python/tree/main/examples/bzlmod).
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A fully-working example without using bzlmod is in [`examples/build_file_generation`](../examples/build_file_generation).
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The following documentation covers using bzlmod.
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## Adding Gazelle to your project
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First, you'll need to add Gazelle to your `MODULE.bazel` file.
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Get the current version of Gazelle from there releases here: https://github.com/bazelbuild/bazel-gazelle/releases/.
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See the installation `MODULE.bazel` snippet on the Releases page:
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https://github.com/bazel-contrib/rules_python/releases in order to configure rules_python.
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You will also need to add the `bazel_dep` for configuration for `rules_python_gazelle_plugin`.
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Here is a snippet of a `MODULE.bazel` file.
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```starlark
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# The following stanza defines the dependency rules_python.
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bazel_dep(name = "rules_python", version = "0.22.0")
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# The following stanza defines the dependency rules_python_gazelle_plugin.
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# For typical setups you set the version.
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bazel_dep(name = "rules_python_gazelle_plugin", version = "0.22.0")
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# The following stanza defines the dependency gazelle.
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bazel_dep(name = "gazelle", version = "0.31.0", repo_name = "bazel_gazelle")
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# Import the python repositories generated by the given module extension into the scope of the current module.
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use_repo(python, "python3_9")
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use_repo(python, "python3_9_toolchains")
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# Register an already-defined toolchain so that Bazel can use it during toolchain resolution.
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register_toolchains(
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"@python3_9_toolchains//:all",
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)
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# Use the pip extension
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pip = use_extension("@rules_python//python:extensions.bzl", "pip")
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# Use the extension to call the `pip_repository` rule that invokes `pip`, with `incremental` set.
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# Accepts a locked/compiled requirements file and installs the dependencies listed within.
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# Those dependencies become available in a generated `requirements.bzl` file.
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# You can instead check this `requirements.bzl` file into your repo.
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# Because this project has different requirements for windows vs other
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# operating systems, we have requirements for each.
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pip.parse(
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name = "pip",
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requirements_lock = "//:requirements_lock.txt",
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requirements_windows = "//:requirements_windows.txt",
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)
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# Imports the pip toolchain generated by the given module extension into the scope of the current module.
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use_repo(pip, "pip")
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```
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Next, we'll fetch metadata about your Python dependencies, so that gazelle can
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determine which package a given import statement comes from. This is provided
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by the `modules_mapping` rule. We'll make a target for consuming this
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`modules_mapping`, and writing it as a manifest file for Gazelle to read.
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This is checked into the repo for speed, as it takes some time to calculate
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in a large monorepo.
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Gazelle will walk up the filesystem from a Python file to find this metadata,
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looking for a file called `gazelle_python.yaml` in an ancestor folder of the Python code.
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Create an empty file with this name. It might be next to your `requirements.txt` file.
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(You can just use `touch` at this point, it just needs to exist.)
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To keep the metadata updated, put this in your `BUILD.bazel` file next to `gazelle_python.yaml`:
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```starlark
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load("@pip//:requirements.bzl", "all_whl_requirements")
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load("@rules_python_gazelle_plugin//manifest:defs.bzl", "gazelle_python_manifest")
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load("@rules_python_gazelle_plugin//modules_mapping:def.bzl", "modules_mapping")
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# This rule fetches the metadata for python packages we depend on. That data is
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# required for the gazelle_python_manifest rule to update our manifest file.
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modules_mapping(
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name = "modules_map",
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wheels = all_whl_requirements,
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)
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# Gazelle python extension needs a manifest file mapping from
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# an import to the installed package that provides it.
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# This macro produces two targets:
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# - //:gazelle_python_manifest.update can be used with `bazel run`
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# to recalculate the manifest
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# - //:gazelle_python_manifest.test is a test target ensuring that
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# the manifest doesn't need to be updated
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gazelle_python_manifest(
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name = "gazelle_python_manifest",
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modules_mapping = ":modules_map",
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# This is what we called our `pip_parse` rule, where third-party
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# python libraries are loaded in BUILD files.
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pip_repository_name = "pip",
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# This should point to wherever we declare our python dependencies
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# (the same as what we passed to the modules_mapping rule in WORKSPACE)
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# This argument is optional. If provided, the `.test` target is very
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# fast because it just has to check an integrity field. If not provided,
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# the integrity field is not added to the manifest which can help avoid
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# merge conflicts in large repos.
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requirements = "//:requirements_lock.txt",
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# include_stub_packages: bool (default: False)
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# If set to True, this flag automatically includes any corresponding type stub packages
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# for the third-party libraries that are present and used. For example, if you have
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# `boto3` as a dependency, and this flag is enabled, the corresponding `boto3-stubs`
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# package will be automatically included in the BUILD file.
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#
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# Enabling this feature helps ensure that type hints and stubs are readily available
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# for tools like type checkers and IDEs, improving the development experience and
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# reducing manual overhead in managing separate stub packages.
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include_stub_packages = True
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)
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```
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Finally, you create a target that you'll invoke to run the Gazelle tool
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with the rules_python extension included. This typically goes in your root
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`/BUILD.bazel` file:
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```starlark
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load("@bazel_gazelle//:def.bzl", "gazelle")
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# Our gazelle target points to the python gazelle binary.
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# This is the simple case where we only need one language supported.
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# If you also had proto, go, or other gazelle-supported languages,
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# you would also need a gazelle_binary rule.
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# See https://github.com/bazelbuild/bazel-gazelle/blob/master/extend.rst#example
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gazelle(
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name = "gazelle",
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gazelle = "@rules_python_gazelle_plugin//python:gazelle_binary",
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)
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```
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That's it, now you can finally run `bazel run //:gazelle` anytime
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you edit Python code, and it should update your `BUILD` files correctly.
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### Directives
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You can configure the extension using directives, just like for other

‎gazelle/docs/index.md‎

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```{toctree}
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:maxdepth: 1
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installation_and_usage
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```

‎gazelle/docs/installation_and_usage.md‎

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# Installation and Usage
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## Example
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Examples of using Gazelle with Python can be found in the `rules_python`
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repo:
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* bzlmod: {gh-path}`examples/bzlmod_build_file_generation`
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* WORKSPACE: {gh-path}`examples/build_file_generation`
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:::{note}
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The following documentation covers using bzlmod.
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:::
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## Adding Gazelle to your project
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First, you'll need to add Gazelle to your `MODULE.bazel` file. Get the current
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version of [Gazelle][bcr-gazelle] from the [Bazel Central Registry][bcr]. Then
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do the same for [`rules_python`][bcr-rules-python] and
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[`rules_python_gazelle_plugin`][bcr-rules-python-gazelle-plugin].
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[bcr-gazelle]: https://registry.bazel.build/modules/gazelle
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[bcr]: https://registry.bazel.build/
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[bcr-rules-python]: https://registry.bazel.build/modules/rules_python
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[bcr-rules-python-gazelle-plugin]: https://registry.bazel.build/modules/rules_python_gazelle_plugin
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Here is a snippet of a `MODULE.bazel` file. Note that most of it is just
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general config for `rules_python` itself - the Gazelle plugin is only two lines
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at the end.
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```starlark
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################################################
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## START rules_python CONFIG ##
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## See the main rules_python docs for details ##
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################################################
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bazel_dep(name = "rules_python", version = "1.5.1")
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python = use_extension("@rules_python//python/extensions:python.bzl", "python")
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python.toolchain(python_version = "3.12.2")
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use_repo(python, "python_3_12_2")
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pip = use_extension("@rules_python//python:extensions.bzl", "pip")
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pip.parse(
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hub_name = "pip",
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requirements_lock = "//:requirements_lock.txt",
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requirements_windows = "//:requirements_windows.txt",
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)
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use_repo(pip, "pip")
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##############################################
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## START rules_python_gazelle_plugin CONFIG ##
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##############################################
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# The Gazelle plugin depends on Gazelle.
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bazel_dep(name = "gazelle", version = "0.33.0", repo_name = "bazel_gazelle")
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# Typically rules_python_gazelle_plugin is version matched to rules_python.
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bazel_dep(name = "rules_python_gazelle_plugin", version = "1.5.1")
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```
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Next, we'll fetch metadata about your Python dependencies, so that gazelle can
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determine which package a given import statement comes from. This is provided
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by the `modules_mapping` rule. We'll make a target for consuming this
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`modules_mapping`, and writing it as a manifest file for Gazelle to read.
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This is checked into the repo for speed, as it takes some time to calculate
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in a large monorepo.
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Gazelle will walk up the filesystem from a Python file to find this metadata,
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looking for a file called `gazelle_python.yaml` in an ancestor folder
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of the Python code. Create an empty file with this name. It might be next
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to your `requirements.txt` file. (You can just use {command}`touch` at
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this point, it just needs to exist.)
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To keep the metadata updated, put this in your `BUILD.bazel` file next
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to `gazelle_python.yaml`:
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```starlark
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# `@pip` is the hub_name from pip.parse in MODULE.bazel.
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load("@pip//:requirements.bzl", "all_whl_requirements")
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load("@rules_python_gazelle_plugin//manifest:defs.bzl", "gazelle_python_manifest")
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load("@rules_python_gazelle_plugin//modules_mapping:def.bzl", "modules_mapping")
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# This rule fetches the metadata for python packages we depend on. That data is
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# required for the gazelle_python_manifest rule to update our manifest file.
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modules_mapping(
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name = "modules_map",
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wheels = all_whl_requirements,
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)
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# Gazelle python extension needs a manifest file mapping from
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# an import to the installed package that provides it.
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# This macro produces two targets:
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# - //:gazelle_python_manifest.update can be used with `bazel run`
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# to recalculate the manifest
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# - //:gazelle_python_manifest.test is a test target ensuring that
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# the manifest doesn't need to be updated
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gazelle_python_manifest(
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name = "gazelle_python_manifest",
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modules_mapping = ":modules_map",
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# This is what we called our `pip.parse` rule in MODULE.bazel, where third-party
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# python libraries are loaded in BUILD files.
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pip_repository_name = "pip",
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# This should point to wherever we declare our python dependencies
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# (the same as what we passed to the modules_mapping rule in WORKSPACE)
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# This argument is optional. If provided, the `.test` target is very
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# fast because it just has to check an integrity field. If not provided,
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# the integrity field is not added to the manifest which can help avoid
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# merge conflicts in large repos.
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requirements = "//:requirements_lock.txt",
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# include_stub_packages: bool (default: False)
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# If set to True, this flag automatically includes any corresponding type stub packages
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# for the third-party libraries that are present and used. For example, if you have
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# `boto3` as a dependency, and this flag is enabled, the corresponding `boto3-stubs`
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# package will be automatically included in the BUILD file.
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# Enabling this feature helps ensure that type hints and stubs are readily available
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# for tools like type checkers and IDEs, improving the development experience and
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# reducing manual overhead in managing separate stub packages.
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include_stub_packages = True
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)
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```
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Finally, you create a target that you'll invoke to run the Gazelle tool
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with the `rules_python` extension included. This typically goes in your root
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`/BUILD.bazel` file:
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```starlark
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load("@bazel_gazelle//:def.bzl", "gazelle", "gazelle_binary")
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gazelle_binary(
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name = "gazelle_multilang",
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languages = [
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# List of language plugins.
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# If you want to generate py_proto_library targets PR #3057), then
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# the proto language plugin _must_ come before the rules_python plugin.
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#"@bazel_gazelle//lanugage/proto",
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"@rules_python_gazelle_plugin//python",
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],
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)
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gazelle(
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name = "gazelle",
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gazelle = ":gazelle_multilang",
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)
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```
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That's it, now you can finally run `bazel run //:gazelle` anytime
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you edit Python code, and it should update your `BUILD` files correctly.

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