Python Launcher for Unix rewrite in pure Python PoC

py.py

import os
import sys


def activated_venv():
    """Check if a virtual environment is activated."""
    if "VIRTUAL_ENV" not in os.environ:
        return None

    os.path.join(os.environ["VIRTUAL_ENV"], "bin", "python")


def preexisting_venv():
    """Check if a virtual environment already exists."""
    working_dir = os.getcwd()
    while True:
        venv_path = os.path.join(working_dir, ".venv", "bin", "python")
        if os.path.exists(venv_path):
            return venv_path

        parent_dir = os.path.dirname(working_dir)
        if parent_dir == working_dir:
            break
        else:
            working_dir = parent_dir

MIN_FILE_LENGTH = len("python3.0")

def _key(python):
    dir, item = python
    version_str = item.removeprefix("python")
    major, _, minor = version_str.partition('.')
    try:
        major = int(major)
    except ValueError:
        return -1, -1
    try:
        minor = int(minor)
    except ValueError:
        return -1, -1

    return major, minor

def path():
    """Search $PATH for the newest Python interpreter."""
    pythons = []
    for dir in os.environ["PATH"].split(os.pathsep):
        if not os.path.exists(dir):
            continue
        pythons.extend([(dir, item)
                    for item in os.listdir(dir)
                        if len(item)>=MIN_FILE_LENGTH and item.startswith("python") and '.' in item])


    newest_python = max(pythons, key=_key)

    return os.path.join(*newest_python)


def exec_(python_path):
    """Execute the found Python interpreter."""
    args = sys.argv[1:]
    os.execv(python_path, [python_path, *args])


if  __name__ == "__main__":
    if (python_path := activated_venv()) is None:
        if (python_path := preexisting_venv()) is None:
            if (python_path := path()) is None:
                raise RuntimeError("No Python interpreter found.")

    exec_(python_path)