Loading rules_python should not depend on known python versions

Tái hiện lỗi bằng bazel run @rules_python//python/bin:repl với cấu hình MODULE.bazel được cung cấp. Bắt đầu với python/extensions:python.bzl và python/private/python_register_toolchains.bzl, đặc biệt là dòng 106 được báo cáo, để theo dõi lý do phiên bản hệ thống được kiểm tra trước phiên bản đã cấu hình. Hoàn tất khi có thể cấu hình một phiên bản python.toolchain hợp lệ mà không xảy ra lỗi do phiên bản Python hệ thống không xác định.

🐞 bug report

Affected Rule

The issue is caused by the rule:

python = use_extension("@rules_python//python/extensions:python.bzl", "python")

Loading python seems to check the system python version. This makes it impossible to use rules_python if the version is not in its version dict:

        File "$BUILD/external/rules_python+/python/private/python_register_toolchains.bzl", line 106, column 31, in python_register_toolchains
                sha256 = tool_versions[python_version]["sha256"].get(platform, None)
Error: key "3.14.0b2" not found in dictionary
ERROR: Analysis of target '@@rules_python+//python/bin:repl' failed; build aborted: 
  error evaluating module extension @@rules_python+//python/extensions:python.bzl%python

Note that this happens when loading the python extension, which is required to configure the python version, creating a catch 22.

Is this a regression?

I have found other occurrences (1,2) of the error message (as below) but I don't think it was traced to this before. It it likely a regression that happens every now and then when some python version gets a weird id and due to the hard coded map of versions in rules_python.

Description

Nothing that depends on rules_python builds. Simplest example
bazel run @rules_python//python/bin:repl

🔬 Minimal Reproduction

This depends on your system python, so it may be best to reproduce in a container. I am running Manjaro with python up to date. The exact python version is 3.14.2 (main, Jan 2 2026, 14:27:39) [GCC 15.2.1 20251112].

Relevant package information (from pacman -Qi python):

Name            : python
Version         : 3.14.2-2
Architecture    : x86_64
Build Date      : Fri 02 Jan 2026 03:27:39 PM CET
Install Date    : Wed 11 Feb 2026 10:13:47 PM CET

Here's my entire python MODULE.bazel setup section (tried older versions of rules_python as well):

bazel_dep(name = "rules_python", version = "1.9.0-rc0")

python = use_extension("@rules_python//python/extensions:python.bzl", "python")
python.toolchain(
    python_version = "3.13",
)
use_repo(python, "python_toolchains")
register_toolchains("@python_toolchains//:all")

🔥 Exception or Error

$ bazel run @rules_python//python/bin:repl 
Starting local Bazel server (8.5.1) and connecting to it...
ERROR: $BUILD/external/rules_python+/python/private/python_register_toolchains.bzl:106:31: Traceback (most recent call last):
        File "$BUILD/external/rules_python+/python/private/python.bzl", line 273, column 53, in _python_impl
                register_result = python_register_toolchains(
        File "$BUILD/external/rules_python+/python/private/python_register_toolchains.bzl", line 106, column 31, in python_register_toolchains
                sha256 = tool_versions[python_version]["sha256"].get(platform, None)
Error: key "3.14.0b2" not found in dictionary
ERROR: Analysis of target '@@rules_python+//python/bin:repl' failed; build aborted: error evaluating module extension @@rules_python+//python/extensions:python.bzl%python

The expectation would be that the unknown python version is ignored as long as a valid python version is configured via python.toolchain(version=$KNOWN_VERSION}). Whatever else depends on the system python version before is quite likely an isolation failure wrt. reproducibility.

🌍 Your Environment

Operating System:
Linux $HOSTNAME 6.18.8-1-MANJARO #1 SMP PREEMPT_DYNAMIC Fri, 30 Jan 2026 23:06:21 +0000 x86_64 GNU/Linux

Output of bazel version:
bazel 8.5.1

Confirmed on 9.0.0 as well

Rules_python version:
1.9.0-rc0, 1.84.0

Anything else relevant?