Windows py_test launcher cannot find bootstrap stub when target name contains slashes

[titusfortner]

🐞 bug report

Affected Rule

py_test / py_binary

Is this a regression?

Yes — v1 (which delegated to Bazel's native Python rules); this is part of our Selenium update to v2 (SeleniumHQ/selenium#17502)

Description

On Windows, the bootstrap stub is declared at python/private/py_executable.bzl:427 using executable.basename, which drops the directory component. The stub is therefore created relative to the package root rather than as a sibling of the .exe launcher. When a target name contains / (so Bazel writes the .exe into a nested output directory), the launcher cannot find the stub next to itself and execution fails.

🔬 Minimal Reproduction

A py_test whose name attribute contains /, e.g.:

  py_test(
      name = "test/unit/foo/color_tests-unit",
      srcs = ["color_tests.py"],
  )

Build on Windows and inspect the action output paths:
stub: bazel-out/x64_windows-fastbuild/bin/pkg/color_tests-unit
executable: bazel-out/x64_windows-fastbuild/bin/pkg/test/unit/foo/color_tests-unit.exe

🔥 Exception or Error

[Errno 2] No such file or directory

🌍 Your Environment

Operating System:

  
Microsoft Windows Server 2025
  

Output of bazel version:

  
9.1.0
  

Rules_python version:

  
2.0.1
  

Anything else relevant?
PR incoming