Dev Guide

This document covers tips and guidance for working on the rules_python code base. Its primary audience is first-time contributors.

Running tests

Running tests is particularly easy thanks to Bazel, simply run:

bazel test //...

And it will run all the tests it can find. The first time you do this, it will probably take a long time because various dependencies will need to be downloaded and set up. Subsequent runs will be faster, but there are many tests, and some of them are slow. If you’re working on a particular area of code, you can run just the tests in those directories instead, which can speed up your edit-run cycle.

Writing Tests

Most code should have tests of some sort. This helps us have confidence that refactors didn’t break anything and that releases won’t have regressions.

We don’t require 100% test coverage; testing certain Bazel functionality is difficult, and some edge cases are simply too hard to test or not worth the extra complexity. We try to judiciously decide when not having tests is a good idea.

Tests go under tests/. They are loosely organized into directories for the particular subsystem or functionality they are testing. If an existing directory doesn’t seem like a good match for the functionality being tested, then it’s fine to create a new directory.

Re-usable test helpers and support code go in tests/support. Tests don’t need to be perfectly factored and not every common thing a test does needs to be factored into a more generally reusable piece. Copying and pasting is fine. It’s more important for tests to balance understandability and maintainability.

Test utilities

General code to support testing is in tests/support. It has a variety of functions, constants, rules etc, to make testing easier. Below are some common utilities that are frequently used.

sh_py_run_test

The sh_py_run_test <tests/support/sh_py_run_test.bzl rule is a helper to make it easy to run a Python program with custom build settings using a shell script to perform setup and verification. This is best to use when verifying behavior needs certain environment variables or directory structures to correctly and reliably verify behavior.

When adding a test, you may find the flag you need to set isn’t supported by the rule. To have it support setting a new flag, see the py_reconfig_test docs below.

py_reconfig_test

The py_reconfig_test and py_reconfig_binary rules are helpers for running Python binaries and tests with custom build flags. This is best to use when verifying behavior that requires specific flags to be set and when the program itself can verify the desired state.

They are located in tests/support/py_reconfig.bzl

When adding a test, you may find the flag you need to set isn’t supported by the rule. To have it support setting a new flag:

Add an attribute to the rule. It should have the same name as the flag it’s for. It should be a string, string_list, or label attribute – this allows distinguishing between if the value was specified or not.
Modify the transition and add the flag to both the inputs and outputs list, then modify the transition’s logic to check the attribute and set the flag value if the attribute is set.

whl_from_dir_repo

The whl_from_dir_repo repository rule in tests/support/whl_from_dir takes a directory tree and turns it into a .whl file. This can be used to create arbitrary whl files to verify functionality.

Integration tests

An integration test is one that runs a separate Bazel instance inside the test. These tests are discouraged unless absolutely necessary because they are slow, require a lot of memory and CPU, and are generally harder to debug. Integration tests are reserved for things that simply can’t be tested otherwise, or for simple high-level verification tests.

Integration tests live in tests/integration. When possible, add to an existing integration test.

Updating internal dependencies

Modify the ./python/private/pypi/requirements.txt file and run:
```
bazel run //private:whl_library_requirements.update
```
Run the following target to update twine dependencies:
```
bazel run //private:requirements.update
```

Bump the coverage dependencies using the script using:

bazel run //tools/private/update_deps:update_coverage_deps <VERSION>
# for example:
# bazel run //tools/private/update_deps:update_coverage_deps 7.6.1

Updating tool dependencies

It’s suggested to routinely update the tool versions within our repo. Some of the tools are using requirement files compiled by uv, and others use other means. In order to have everything self-documented, we have a special target, //private:requirements.update, which uses rules_multirun to run all of the requirement-updating scripts in sequence in one go. This can be done once per release as we prepare for releases.

Creating Backport PRs

The steps to create a backport PR are:

Create an issue for the patch release; use the patch release template.
Create a fork of rules_python.
Checkout the release/X.Y branch.
Use git cherry-pick -x to cherry pick the desired fixes.
Update the release’s CHANGELOG.md file:
- Add a Major.Minor.Patch section if one doesn’t exist
- Copy the changelog text from main to the release’s changelog.
Send a PR with the backport’s changes.
- The title should be backport: PR#N to Major.Minor
- The body must preserve the original PR’s number, commit hash, description, and authorship. Use the following format (git cherry-pick will use this format):
```
<original PR title>

<original PR body>
(cherry picked from commit <commit hash>)
-----
Co-authored-by: <original PR author; separate lines for each>
```
- If the PR contains multiple backport commits, separate each’s description with -----.
Send a PR to update the main branch’s CHANGELOG.md to reflect the changes done in the patched release.