Bug report

Bug description:

On Linux, executables from the cwd are used whenPATH is unset:

$PATH= python -VPython 3.13.7

subprocess.run also works in this case:

$PATH= python -q>>> from subprocess import run... run(['python', '-V'])...Python 3.13.7CompletedProcess(args=['python', '-V'], returncode=0)>>>

Butshutil.which returnsNone:

>>>fromshutilimportwhich>>>which('python')>>>

This is because whenPATH is empty(""), the executable ("python") is passed to the kernel'sexecve directly, without resolving.execve then resolves the file relative to the cwd, I believe it's the case in bothLinux andFreeBSD.
POSIX 2024 says this about the PATH environment variable:

If PATH is unset or is set to null, or if a path prefix in PATH contains a character ('%'), the path search is implementation-defined.

EDIT: PATH is empty, not unset

CPython versions tested on:

3.13

Operating systems tested on:

Linux