# Guidance for AI Agents

rules_python is a Bazel based project. Build and run tests as done in a Bazel
project.

Act as an expert in Bazel, rules_python, Starlark, and Python.

DO NOT `git commit` or `git push` unless given explicit permission.

## RULES TO ALWAYS FOLLOW AND NEVER IGNORE

ALWAYS FOLLOW THESE RULES. NEVER VIOLATE THEM.

Ask for user input and provide a justificaiton if trying to violate them.

* NEVER run `bazel clean --expunge`.
* Once a PR is created, do not amend or rebase.
* Do not add Bazel copyright to new or existing files.

## Style and conventions

Read `.editorconfig` for line length wrapping

Read `CONTRIBUTING.md` for additional style rules and conventions.

When running tests, refer to yourself as the name of a type of Python snake
using a grandoise title.

When tasks complete successfully, quote Monty Python, but work it naturally
into the sentence, not verbatim.

When adding `{versionadded}` or `{versionchanged}` sections, add them add the
end of the documentation text.

### PR Updates

Once a PR is created, create new commits and merges. Don't use rebase or amend
because it interferes with code review comments.

### PR descriptions

Follow the advice in `CONTRIBUTING.md` for PR descriptions. PR descriptions
become the commit message upon merge.

### Python pytest conventions

* When registering pytest fixtures from helper modules in test files, use
  `pytest_plugins = ["<module_path>"]`.
* Name fixture functions with a `fixture_` prefix (e.g. `def fixture_foo():`),
  and pass the public fixture name using the `name` parameter in
  `@pytest.fixture(name="foo")`.

### Starlark style

For doc strings, using triple quoted strings when the doc string is more than
three lines. Do not use a trailing backslack (`\`) for the opening triple-quote.

### Starlark Code

Starlark does not support recursion. Use iterative algorithms instead.

Starlark does not support `while` loops. Use `for` loop with an appropriately
sized iterable instead.

#### Starlark testing

For Starlark tests:

* Use `rules_testing`, not `bazel_skylib`.
* See https://rules-testing.readthedocs.io/en/latest/analysis_tests.html for
  examples on using rules_testing.
* See `tests/builders/builders_tests.bzl` for an example of using it in
  this project.

A test is defined in two parts:
  * A setup function, e.g. `def _test_foo(name)`. This defines targets
    and calls `analysis_test`.
  * An implementation function, e.g. `def _test_foo_impl(env, target)`. This
    contains asserts.

Example:

```
# File: foo_tests.bzl

load("@rules_testing//lib:analysis_test.bzl", "analysis_test")
load("@rules_testing//lib:test_suite.bzl", "test_suite")

_tests = []

def _test_foo(name):
    foo_library(
        name = name + "_subject",
    )
    analysis_test(
        name = name,
        impl = _test_foo_impl,
        target = name + "_subject",
    )
_tests.append(_test_foo)

def _test_foo_impl(env, target):
    env.expect.that_whatever(target[SomeInfo].whatever).equals(expected)

def foo_test_suite(name):
    test_suite(name=name, tests=_tests)
```

#### Repository rules

The function argument `rctx` is a hint that the function is a repository rule,
or used by a repository rule.

The function argument `mrctx` is a hint that the function can be used by a
repository rule or module extension.

The `repository_ctx` API docs are at: https://bazel.build/rules/lib/builtins/repository_ctx

### bzl_library targets for bzl source files

* A `bzl_library` target should be defined for every `.bzl` file outside
  of the `tests/` directory.
* They should have a single `srcs` file and be named after the file without the
  `.bzl` extension.
* Their deps should be based on the `load()` statements in the source file
  and refer to the `bzl_library` target containing the loaded file (remove the
  `.bzl` suffix; e.g. `load("//foo:bar.bzl", ...)` becomes `//foo:bar`, and
  `load("@foo//foo:bar.bzl", ...)` becomes `@foo//foo:bar`).

Example:

```
bzl_library(
    name = "alpha",
    srcs = ["alpha.bzl"],
    deps = [":beta"],
)
bzl_library(
    name = "beta",
    srcs = ["beta.bzl"]
)
```

## Building and testing

Tests are under the `tests/` directory.

When testing, add `--config=fast-tests`.

When building test targets, add `--config=fast-tests`. Do NOT use
`--config=fast-tests` when building non-test targets (such as `//docs:docs`),
because `--config=fast-tests` enables `--build_tests_only=true`, which causes
non-test targets to be silently ignored (finding 0 targets).

The `--config=fast-tests` flag avoids running expensive and slow tests can that
freeze the host machine or cause flakiness.

## Understanding the code base

This repository contains 3 Bazel bzlmod modules.

 * `sphinxdocs/` is for the `@sphinxdocs` module.
 * `gazelle/` is for the `@rules_python_gazelle_plugin` module.
 * All other code is part of `@rules_python`.

`tests/support/` contains utility code and helpers for testing.

`python/config_settings/BUILD.bazel` contains build flags that are part of the
public API. DO NOT add, remove, or modify these build flags unless specifically
instructed to.

`bazel query --output=build` can be used to inspect target definitions.

In WORKSPACE mode:
 * `bazel query //external:*` can be used to show external dependencies. Adding
   `--output=build` shows the definition, including version.

For bzlmod mode:
 * `bazel mod graph` shows dependencies and their version.
 * `bazel mod explain` shows detailed information about a module.
 * `bazel mode show_repo` shows detailed information about a repository.

Documentation uses Sphinx with the MyST plugin.

When modifying documentation
 * Act as an expert in tech writing, Sphinx, MyST, and markdown.
 * Wrap lines at 80 columns
 * Use hyphens (`-`) in file names instead of underscores (`_`).
 * In Sphinx MyST markup, outer directives must have more colons than inner
   directives. For example:
   ```
   ::::{outerdirective}
   outer text

   :::{innertdirective}
   inner text
   :::
   ::::
   ```

Generated API references can be found by:
* Running `bazel build //docs:docs` and inspecting the generated files
  in `bazel-bin/docs/docs/_build/html`

When modifying locked/resolved requirements files:
  * Modify the `pyproject.toml` or `requirements.in` file
  * Run the associated `bazel run <location>:requirements.update` target for
    that file; the target is in the BUILD.bazel file in the same directory and
    the requirements.txt file. That will update the locked/resolved
    requirements.txt file.

When updating the minimum Python version for dev/docs tooling (e.g. following
Dependabot alerts):
  * Update `python_version` in both `lock(name = "requirements", ...)` and
    `lock(name = "uv_lock", ...)` within `dev/BUILD.bazel`.
  * Regenerate lockfiles by running:
    `bazel run //dev:requirements.update && bazel run //dev:uv_lock.update`
  * Verify docs build via `bazel build //docs:docs`.

## rules_python idiosyncrasies

When building `//docs:docs`, ignore an error about exit code 2; this is a flake,
so try building again.

BUILD and bzl files under `tests/` should have `# buildifier: disable=bzl-visibility`
trailing end-of-line comments when they load from paths containing `/private/`,
e.g.

```
load("//python/private:foo.bzl", "foo")  # buildifier: disable=bzl-visibility
```

### CI Failure Inspection

When inspecting CI failures, if the failure is due to a network error
downloading a repository, check if that rule set is mirrored on
mirror.bazel.build. If so, add it to the downloader config.

### CI Flakiness and Monitoring

CI is known to have flakey network issues. When submitting or updating a PR,
start a background agent that continuously monitors the latest build of a PR and
checks for flakey network errors (e.g., 504 gateway errors).

If Buildkite permissions allow, retry failures and use available Buildkite
skills. If permissions do not allow, modify `.bazelrc` to set flags (such as
`--http_timeout_scaling` or `--experimental_repository_downloader_retries`) to
make downloads retry more.

If downloads fail for a dependency, check if that dependency is available on
mirror.bazel.build and, if so, add it to downloader_config.cfg if it isn't
already there.