[Spikhalskiy]

Python version transitions also reach native dependencies that don't use Python, so a C++ or Rust library under bindings for several Python versions is configured once per version. Add a root-module config.python_version_scope(scope = "project") tag that gives //python/config_settings:python_version Bazel's project scope. Under project scope the public flag label becomes an alias to a flag in the generated config repository, whose PROJECT.scl delegates ownership to the consumer's //:PROJECT.scl through an actual alias. Targets outside it see the command-line version and share one configuration. Bazel reads the project file of every top-level target's package, so only the opt-in ships that alias, and other builds need no root PROJECT.scl. Universal scope stays the default, and tags from dependency modules are ignored.

Project scope requires Bazel 9.2 or later, the first release that keeps project-scoped flags across exec transitions (bazelbuild/bazel#28320). Owned exec tools such as the precompiler keep the selected version, which preserves the requirement behind #4335. Older Bazel versions fail with an explicit error.

aspect_rules_py terminals set this flag as well as their own; the matching opt-in there is aspect-build/rules_py#1650.

An integration test covers target and exec propagation, convergence of the native dependency's configuration, the unchanged default, and ignored dependency-module tags.

Fixes #4375

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has insufficient remaining quota.

An organization owner can choose the repository organization as the billing source for reviews requested by members with a paid Copilot license. Learn about Copilot code review billing.

[aignas]

A few questions, but in general LGTM.

[aignas]

What about repositories that are created due to the pip.parse tag class? Does the @@rules_python+ means that those will be also included?

[aignas]

probe.bzl is a very abstract name. Is there a way to make it more concrete?

[rickeylev]

Fabian has also brought up a desire to reign in config state explosion. The basic problem with using PROJECTS.scl is it means every end-user bazel project has to configure their own.

Would one of these options work better than this?

• For the data attribute, reset all the python flags to their default
• Provide a reset_python_flags() rule you can apply to targets e.g. to wrap around a cc library that is Python-agnostic.

It's somewhat tricky because a handful of flags matter, but not always (e.g. compiling LLVM doesn't care about python, but compiling a python C extension does).

[rickeylev]

"such as the precompiler" -- is the precompiler ending up in multiple configs one of the motivations for this PR?

[aignas]

The basic problem with using PROJECTS.scl is it means every end-user bazel project has to configure their own.

Good catch, that had not occured to me.