# How to contribute

We'd love to accept your patches and contributions to this project. There are
just a few small guidelines you need to follow.

## Contributor License Agreement

First, the most important step: signing the Contributor License Agreement. We
cannot look at any of your code unless one is signed.

Contributions to this project must be accompanied by a Contributor License
Agreement. You (or your employer) retain the copyright to your contribution,
this simply gives us permission to use and redistribute your contributions as
part of the project. Head over to <https://cla.developers.google.com/> to see
your current agreements on file or to sign a new one.

You generally only need to submit a CLA once, so if you've already submitted one
(even if it was for a different project), you probably don't need to do it
again.

## Getting started

Before we can work on the code, we need to get a copy of it and setup some
local environment and tools.

First, fork the code to your user and clone your fork. This gives you a private
playground where you can do any edits you'd like. For this guide, we'll use
the [GitHub `gh` tool](https://github.com/cli/cli)
([Linux install](https://github.com/cli/cli/blob/trunk/docs/install_linux.md)).
(More advanced users may prefer the GitHub UI and raw `git` commands).

```shell
gh repo fork bazel-contrib/rules_python --clone --remote
```

Next, make sure you have a new enough version of Python installed that supports the
various code formatters and other devtools. For a quick start,
[install pyenv](https://github.com/pyenv/pyenv-installer) and
at least Python 3.9.15:

```shell
curl https://pyenv.run | bash
pyenv install 3.9.15
pyenv shell 3.9.15
```

## Development workflow

It's suggested that you create what is called a "feature/topic branch" in your
fork when you begin working on code you want to eventually send or code review.

```
git checkout main # Start our branch from the latest code
git checkout -b my-feature # Create and switch to our feature branch
git push origin my-feature # Cause the branch to be created in your fork.
```

From here, you then edit code and commit to your local branch. If you want to
save your work to github, you use `git push` to do so:

```
git push origin my-feature
```

Once the code is in your github repo, you can then turn it into a Pull Request
to the actual rules_python project and begin the code review process.

## Developer guide

For more more details, guidance, and tips for working with the code base,
see [docs/devguide.md](./devguide)

## Formatting

Starlark files should be formatted by
[buildifier](https://github.com/bazelbuild/buildtools/blob/master/buildifier/README.md).
Otherwise the Buildkite CI will fail with formatting/linting violations.
We suggest using a pre-commit hook to automate this.
First [install pre-commit](https://pre-commit.com/#installation),
then run

```shell
pre-commit install
```

### Running buildifer manually

You can also run buildifier manually. To do this,
[install buildifier](https://github.com/bazelbuild/buildtools/blob/master/buildifier/README.md),
and run the following command:

```shell
$ buildifier --lint=fix --warnings=native-py -warnings=all WORKSPACE
```

Replace the argument "WORKSPACE" with the file that you are linting.

## Code reviews

All submissions, including submissions by project members, require review. We
use GitHub pull requests for this purpose. Consult [GitHub Help] for more
information on using pull requests.

[GitHub Help]: https://help.github.com/articles/about-pull-requests/

### Commit messages and PR descriptions

Commit messages (upon merging) and PR messages should follow the [Conventional
Commits](https://www.conventionalcommits.org/) style:

```
type(scope)!: <summary>

<body>

BREAKING CHANGE: <summary>
```

Where `(scope)` is optional, and `!` is only required if there is a breaking change.
If a breaking change is introduced, then `BREAKING CHANGE:` is required; see
the [Breaking Changes](#breaking-changes) section for how to introduce breaking
changes.

User visible changes, such as features, fixes, or notable refactors, should
be documneted in CHANGELOG.md and their respective API doc. See [Documenting
changes] for how to do so.

Common `type`s:

* `build:` means it affects the building or development workflow.
* `docs:` means only documentation is being added, updated, or fixed.
* `feat:` means a user-visible feature is being added. See [Documenting version
  changes] for how to documenAdd `{versionadded}`
  to appropriate docs.
* `fix:` means a user-visible behavior is being fixed. If the fix is changing
  behavior of a function, add `{versionchanged}` to appropriate docs, as necessary.
* `refactor:` means some sort of code cleanup that doesn't change user-visible
  behavior. Add `{versionchanged}` to appropriate docs, as necessary.
* `revert:` means a prior change is being reverted in some way.
* `test:` means only tests are being added.

For the body, follow this guidance:

* Briefly tells *why* the change is being made. This usually means
  briefly describing how a bug manifests or what can't be accomplished
  without the feature.
* Briefly gives an overview of *how* the code is changed. This is to
  orient readers for the diff they're about to read and understand; it's
  not a verbatim description of what changed.
* List unrelated or notable dev-only changes at the end. e.g. formatting an
  old file, cleaning up testing, adding test support code, etc.

For the full details of types, see
[Conventional Commits](https://www.conventionalcommits.org/).

#### PR description example

```
fix(pypi): handle files with .exe extensions

Currently, if a file with `.exe` is seen, an error
occurs because validation assumes unix-only filenames.
This prevents using whls with pre-built .exe files in
their data payload.

To fix, detect the target OS and use an OS-appropriate
validation function.

* Also adds test helpers for detecting the current OS
```

### Documenting changes

Changes are documented in two places: CHANGELOG.md and API docs.

CHANGELOG.md contains a brief, human friendly, description. This text is
intended for easy skimming so that, when people upgrade, they can quickly get a
sense of what's relevant to them.

API documentation are the doc strings for functions, fields, attributes, etc.
When user-visible or notable behavior is added, changed, or removed, the
`{versionadded}`, `{versionchanged}` or `{versionremoved}` directives should be
used to note the change. When specifying the version, use the values
`VERSION_NEXT_FEATURE` or `VERSION_NEXT_PATCH` to indicate what sort of
version increase the change requires.

These directives use Sphinx MyST syntax, e.g.

```
:::{versionadded} VERSION_NEXT_FEATURE
The `allow_new_thing` arg was added.
:::

:::{versionchanged} VERSION_NEXT_PATCH
Large numbers no longer consume exponential memory.
:::

:::{versionremoved} VERSION_NEXT_FEATURE
The `legacy_foo` arg was removed
:::
```

## Style and idioms

For the most part, we just accept whatever the code formatters do, so there
isn't much style to enforce.

Some miscellanous style, idioms, and conventions we have are:

### Markdown/Sphinx Style

* Use colons for prose sections of text, e.g. `:::{note}`, not backticks.
* Use backticks for code blocks.
* Max line length: 100.

### BUILD/bzl Style

* When a macro generates public targets, use a dot (`.`) to separate the
  user-provided name from the generted name. e.g. `foo(name="x")` generates
  `x.test`. The `.` is our convention to communicate that it's a generated
  target, and thus one should look for `name="x"` when searching for the
  definition.
* The different build phases shouldn't load code that defines objects that
  aren't valid for their phase. e.g.
  * The bzlmod phase shouldn't load code defining regular rules or providers.
  * The repository phase shouldn't load code defining module extensions, regular
    rules, or providers.
  * The loading phase shouldn't load code defining module extensions or
    repository rules.
  * Loading utility libraries or generic code is OK, but should strive to load
    code that is usable for its phase. e.g. loading-phase code shouldn't
    load utility code that is predominately only usable to the bzlmod phase.
* Providers should be in their own files. This allows implementing a custom rule
  that implements the provider without loading a specific implementation.
* One rule per file is preferred, but not required. The goal is that defining an
  e.g. library shouldn't incur loading all the code for binaries, tests,
  packaging, etc; things that may be niche or uncommonly used.
* Separate files should be used to expose public APIs. This ensures our public
  API is well defined and prevents accidentally exposing a package-private
  symbol as a public symbol.

  :::{note}
  The public API file's docstring becomes part of the user-facing docs. That
  file's docstring must be used for module-level API documentation.
  :::
* Repository rules should have name ending in `_repo`. This helps distinguish
  them from regular rules.
* Each bzlmod extension, the "X" of `use_repo("//foo:foo.bzl", "X")` should be
  in its own file. The path given in the `use_repo()` expression is the identity
  Bazel uses and cannot be changed.

## Generated files

Some checked-in files are generated and need to be updated when a new PR is
merged:

* **requirements lock files**: These are usually generated by a
  `compile_pip_requirements` update target, which is usually in the same directory.
  e.g. `bazel run //docs:requirements.update`

## Binary artifacts

Checking in binary artifacts is not allowed. This is because they are extremely
problematic to verify and ensure they're safe. This is true even in
test contexts.

Examples include, but aren't limited to: prebuilt binaries, shared libraries,
zip files, or wheels.

See the dev guide for utilities to help with testing.

(breaking-changes)=
## Breaking Changes

Breaking changes are generally permitted, but we follow a 3-step process for
introducing them. The intent behind this process is to balance the difficulty of
version upgrades for users, maintaining multiple code paths, and being able to
introduce modern functionality.

The general process is:

1. In version `N`, introduce the new behavior, but it must be disabled by
   default. Users can opt into the new functionality when they upgrade to
   version `N`, which lets them try it and verify functionality.
2. In version `N+1`, the new behavior can be enabled by default. Users can
   opt out if necessary, but doing so causes a warning to be issued.
3. In version `N+2`, the new behavior is always enabled and cannot be opted out
   of. The API for the control mechanism can be removed in this release.

Note that the `+1` and `+2` releases are just examples; the steps are not
required to happen in immediately subsequent releases.

Once The first major version is released, the process will be:
1. In `N.M.0` we introduce the new behaviour, but it is disabled by a feature flag.
2. In `N.M+1.0` we may choose the behaviour to become the default if it is not too
   disruptive.
3. In `N+1.0.0` we get rid of the old behaviour.

### How to control breaking changes

The details of the control mechanism will depend on the situation. Below is
a summary of some different options.

* Environment variables are best for repository rule behavior. Environment
  variables can be propagated to rules and macros using the generated
  `@rules_python_internal//:config.bzl` file.
* Attributes are applicable to macros and regular rules, especially when the
  behavior is likely to vary on a per-target basis.
* [User defined build settings](https://bazel.build/extending/config#user-defined-build-settings)
  (aka custom build flags) are applicable for rules when the behavior change
  generally wouldn't vary on a per-target basis. They also have the benefit that
  an entire code base can have them easily enabled by a bazel command line flag.
* Allowlists allow a project to centrally control if something is
  enabled/disabled. Under the hood, they are basically a specialized custom
  build flag.

Note that attributes and flags can seamlessly interoperate by having the default
controlled by a flag, and an attribute can override the flag setting. This
allows a project to enable the new behavior by default while they work to fix
problematic cases to prepare for the next upgrade.

### What is considered a breaking change?

Precisely defining what constitutes a breaking change is hard because it's
easy for _someone, somewhere_ to depend on _some_ observable behavior, despite
our best efforts to thoroughly document what is or isn't supported and hiding
any internal details.

In general, something is considered a breaking change when it changes the
direct behavior of a supported public API. Simply being able to observe a
behavior change doesn't necessarily mean it's a breaking change.

Long standing undocumented behavior is a large grey area and really depends on
how load-bearing it has become and what sort of reasonable expectation of
behavior there is.

Here's some examples of what would or wouldn't be considered a breaking change.

Breaking changes:
  * Renaming an function argument for public functions.
  * Enforcing stricter validation than was previously required when there's a
    sensible reason users would run afoul of it.
  * Changing the name of a public rule.

Not breaking changes:
  * Upgrading dependencies
  * Changing internal details, such as renaming an internal file.
  * Changing a rule to a macro.

## AI-assisted Contributions

Contributions assisted by AI tools are allowed. However, the human author
submitting the pull request is responsible for the contributed code as if they
had written it entirely themselves. This means:

*   **Understanding the code:** You must be able to explain what the code does
    and why it's implemented that way. This includes discussing its
    implications, and any trade-offs made during its development, just as if you
    had written it entirely yourself.
*   **Vetting the correctness and functionality:** You are responsible for
    thoroughly testing and verifying that the code is correct, functional, and
    meets all project requirements and standards.

If the human PR author cannot fulfill these responsibilities, the `rules_python`
maintainers will not spend time reviewing or merging the PR. The goal is to
ensure that all contributions, regardless of their origin, maintain the quality
and integrity of the project and do not place an undue burden on maintainers.

## FAQ

### Installation errors when during `git commit`

If you did `pre-commit install`, various tools are run when you do `git commit`.
This might show as an error such as:

```
[INFO] Installing environment for https://github.com/psf/black.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ...
```

To fix, you'll need to figure out what command is failing and why. Because these
are tools that run locally, its likely you'll need to fix something with your
environment or the installation of the tools. For Python tools (e.g. black or
isort), you can try using a different Python version in your shell by using
tools such as [pyenv](https://github.com/pyenv/pyenv).