🐞 bug report

Affected Rule

The issue is caused by the rule: the python bzlmod extension, when python.single_version_platform_override registers a runtime under a custom platform name. The failure is in the alias repos it generates (@python_X_Y, @python_X_Y_Z), in python/private/toolchain_aliases.bzl.

Is this a regression?

No. It is already present in 1.5.0, the release that added custom platform names (#2880). I also see it in 2.4.6 and on main at 062a81c.

Description

single_version_platform_override can register a runtime under a custom platform name and gate it with target_settings, as the "Registering custom runtimes" docs describe. Once a Python version has such a runtime, every alias target of that version (@python_3_14_4//:python_headers, :files, :python3, and so on) fails to analyze. It fails with the flag set and without it.

toolchain_aliases() builds the select() of each alias from all the platform names of the version, but it creates config settings only for the names in PLATFORMS (python/versions.bzl). The custom name ends up as a select() key with no config setting behind it.

🔬 Minimal Reproduction

https://github.com/vadikmironov/rules_python-custom-platform-alias-repro

bazel build //:python_headers

//:python_headers is an alias of @python_3_14_4//:python_headers. The relevant part of MODULE.bazel:

python = use_extension("@rules_python//python/extensions:python.bzl", "python")
python.toolchain(python_version = "3.14.4")
python.single_version_platform_override(
    platform = "linux-x86-stripped",
    python_version = "3.14.4",
    sha256 = "fe9a9c32d13870af632cbac3dfc7528ae53597e94472aa4c7d6a42e8166136cd",
    target_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_settings = ["@@//:is_stripped_runtime"],
    urls = ["https://github.com/astral-sh/python-build-standalone/releases/download/20260414/cpython-3.14.4+20260414-x86_64-unknown-linux-gnu-install_only_stripped.tar.gz"],
)
use_repo(python, "python_3_14_4")

The build completes when the single_version_platform_override call is removed.

🔥 Exception or Error

ERROR: .../external/rules_python++python+python_3_14_4/BUILD.bazel:32:18: @@rules_python++python+python_3_14_4//:linux-x86-stripped is not a valid select() condition for @@rules_python++python+python_3_14_4//:python_headers.
ERROR: Analysis of target '//:python_headers' failed; build aborted: Analysis failed

🌍 Your Environment

Operating System:

  
Debian 13 (trixie) on WSL2, x86_64
  

Output of bazel version:

  
Bazelisk version: v1.29.0
Build label: 9.3.2
  

Rules_python version:

  
2.4.6. Also main at 062a81c4, and 1.5.0 (with Python 3.13.3, Bazel 9.3.2 and 8.4.2).
  

Anything else relevant?

I ran into this while writing the test for #4471. The existing //tests/toolchains:custom_platform_toolchain_test does not hit it, because its custom platform is on 3.13.1 and nothing builds @python_3_13_1//.... With a custom platform on the newest 3.14, //tests/toolchains:build_test fails, because it builds @python_3_14//:python_headers.

Limiting the select() to the names that have a config setting makes the repro build. But then, with the flag set, the alias points at the regular runtime and not at the custom one.