pdm run does not parse the requested python version correctly

[xulongwu4]

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

1. Create a fresh project with python3.7 as the interpreter

$ mkdir -p project
$ cd project
$ pdm init

2. Try invoking the python interpreter with different version numbers

$ pdm run python3
Python 3.7.12 (default, Sep 26 2021, 21:12:22)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

$ pdm run python3.10
Python 3.7.12 (default, Sep 26 2021, 21:12:22)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

$ pdm run python3.20
Python 3.7.12 (default, Sep 26 2021, 21:12:22)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Actual behavior

No matter what version I request with pdm run python3.X, pdm always pick the selected python3.7 as the interpreter.

Expected behavior

If python3.X is not configured for the current project, I would expect pdm run python3.X to fall back to search in the system PATH variable. Thus if I do pdm run python3.10, it should give me python3.10 in the system PATH, and if I do pdm run python3.20, it should produce an error message like the following:

[PdmUsageError]: Command 'python3.20' is not found on your PATH.

This way, the handling of python interpreter would be consistent with the handling of other executables by pdm run.

Possible causes

I believe the cause if the usage of splitext in parsing the command sent to pdm run in pdm/models/environment.py:

python = os.path.splitext(command)[0]
version = python[6:]

The first line discards the version string after the first dot, so python3.20 would be parsed as python3 going forward. I think this is a bug.

Environment Information

# Paste the output of `pdm info && pdm info --env` below:

$ pdm info
PDM version:
  2.1.4
Python Interpreter:
  /usr/bin/python3.7 (3.7)
Project Root:
  /home/oulongwu/tmp/project
Project Packages:
  /home/oulongwu/tmp/project/__pypackages__/3.7


$ pdm info --env
{
  "implementation_name": "cpython",
  "implementation_version": "3.7.12",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "5.15.68-218.current",
  "platform_system": "Linux",
  "platform_version": "#1 SMP PREEMPT Sat Sep 17 01:51:42 UTC 2022",
  "python_full_version": "3.7.12",
  "platform_python_implementation": "CPython",
  "python_version": "3.7",
  "sys_platform": "linux"
}