Cannot run native python binary script without C++ toolchain

[hoppe-and-dreams]

🐞 bug report

Affected Rule

py_binary

Is this a regression?

Not tested, unlikely.

Description

A trivial pure-Python py_binary appears to require C++ toolchain resolution even though it does not produce any C++ compile or link actions.

In a minimal example, bazel cquery //:hello_world --toolchain_resolution_debug='.*' shows that Bazel resolves both:

• @@bazel_tools//tools/python:toolchain_type
• @@bazel_tools//tools/cpp:toolchain_type

However, bazel aquery //:hello_world only shows Python-related actions such as Python bytecode precompilation and bootstrap/template generation. There are no C++ compile or link actions for the target.

This is surprising for Python-only projects. In environments where no C++ toolchain is configured or available, this can cause a trivial py_binary to fail analysis even though the target itself does not build native code.

From a user perspective, I would expect a pure py_binary with no native extensions and no C/C++ deps to avoid requiring successful C++ toolchain resolution, or at least for the requirement to be documented if it is intentional.

🔬 Minimal Reproduction

MODULE.bazel

module(
    name = "bazel_hello_py",
    version = "0.1.0",
)

bazel_dep(name = "rules_shell", version = "0.6.1")
bazel_dep(name = "rules_python", version = "1.7.0")

BUILD.bazel

load("@rules_python//python:defs.bzl", "py_binary")

py_binary(
    name = "hello_world",
    srcs = ["hello_world.py"],
)

hello_world.py

print("hello world")

bazel run //:hello_world --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=<0|1>
bazel cquery //:hello_world --toolchain_resolution_debug='.*'
bazel aquery //:hello_world

Observed behavior:

1. bazel run //:hello_world succeeds if the repo_env is set to 0 but fails on 1
2. cquery shows that @@bazel_tools//tools/cpp:toolchain_type is resolved for the target.
3. aquery shows only Python-related actions and no C++ compile/link actions.

This suggests the C++ toolchain resolution is happening even for a pure-Python binary with no actual C++ build work.

🔥 Exception or Error

$ bazel run  //:hello_world --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
ERROR: .../external/bazel_tools/src/tools/launcher/BUILD:75:10: While resolving toolchains for target @@bazel_tools//src/tools/launcher:launcher_maker (04bf134): No matching toolchains found for types:
  @@bazel_tools//tools/cpp:toolchain_type
To debug, rerun with --toolchain_resolution_debug='@@bazel_tools//tools/cpp:toolchain_type'
For more information on platforms or toolchains see https://bazel.build/concepts/platforms-intro.
ERROR: Analysis of target '//:hello_world' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.107s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

🌍 Your Environment

Operating System:

  
     Linux (Ubuntu LTS 24.04)
  

Output of bazel version:

  
    Bazelisk version: v1.29.0 Build label: 9.2.0 Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer Build time: Mon Jul 13 18:15:04 2026 (1783966504) Build timestamp: 1783966504 Build timestamp as int: 1783966504
  

Rules_python version:

  
     1.7.0
  

Anything else relevant?

Related issues I found while searching:
rules_python #1857 discusses py_binary and C++ toolchain resolution in cross-compilation scenarios.
rules_python #2500 discusses replacing the stage1 bootstrap with a native launcher.

My main question is whether this C++ toolchain resolution for a pure py_binary is intentional.

[rickeylev]

No, a c++ toolchain isn't supposed to be required.

This bug sounds familiar. I seem to recall a bug where the "launcher maker" thing was being requested even though it wasn't used (it's only used on windows, where a prebuilt binary is used).

[hoppe-and-dreams]

With rules_sh the situation was similar. A simple script did not run without the launcher which need C++. But this behavior was changed from Bazel 8 onwards (on linux).