Pass sys.executable to uv venv if it is the same version (#40)

* Pass sys.executable to `uv venv` if it is the same version

Support the case when pythonX.Y is not in the search path, but
it corresponds to the currently running Python interpreter.

This may happen if e.g. Tox is installed (along with tox-uv) in
a virtual environment that has been created with a Python interpreter
coming from a non-standard location, and then `tox` is invoked
directly from the virtual environment's `bin/` directory:

    [roam@straylight ~]$ mkdir ~/not-in-path

    [roam@straylight ~]$ uv venv -p /opt/some-vendor/python3/bin/python3 ~/not-in-path/venv
    Using Python 3.8.17 interpreter at: /opt/some-vendor/python3/bin/python3
    Creating virtualenv at: /home/roam/not-in-path/venv
    Activate with: source /home/roam/not-in-path/venv/bin/activate

    [roam@straylight ~]$ (set -e; . ~/not-in-path/venv/bin/activate; uv pip install tox tox-uv)
    ...snip...

    [roam@straylight ~]$ printf -- '%s\n' '[testenv]' 'package=skip' 'commands=python3 -c "print()"' > ~/not-in-path/tox.ini

    [roam@straylight ~]$ ~/not-in-path/venv/bin/tox -c ~/not-in-path/tox.ini
    py: venv /home/roam/not-in-path> venv/bin/uv venv -p 3.8 /home/roam/not-in-path/.tox/py/.venv
      × No Python 3.8 in `PATH`. Is Python 3.8 installed?
    py: exit 1 (0.01 seconds) /home/roam/not-in-path> venv/bin/uv venv -p 3.8 /home/roam/not-in-path/.tox/py/.venv pid=1059253
      py: FAIL code 1 (0.02 seconds)
      evaluation failed :( (0.11 seconds)

    [roam@straylight ~]$

* Let Ruff know that we try to use subprocess responsibly

* Tox is certainly installed during the test suite run

src/tox_uv/_venv.py

@@ -113,7 +113,13 @@ def _default_pass_env(self) -> list[str]:
 
     def create_python_env(self) -> None:
         base = self.base_python.version_info
-        version_spec = f"{base.major}.{base.minor}" if base.minor else f"{base.major}"
+        version_spec = (
+            sys.executable
+            if (base.major, base.minor) == sys.version_info[:2]
+            else f"{base.major}.{base.minor}"
+            if base.minor
+            else f"{base.major}"
+        )
         cmd: list[str] = [self.uv, "venv", "-p", version_spec]
         if self.options.verbosity > 2:  # noqa: PLR2004
             cmd.append("-v")

tests/test_tox_uv_venv.py

@@ -1,5 +1,10 @@
 from __future__ import annotations
 
+import importlib.util
+import os
+import os.path
+import pathlib
+import subprocess  # noqa: S404
 import sys
 from configparser import ConfigParser
 from typing import TYPE_CHECKING
@@ -86,3 +91,28 @@ def test_uv_env_site_package_dir(tox_project: ToxProjectCreator) -> None:
     else:  # pragma: win32 no cover
         path = str(env_dir / "lib" / f"python{ver.major}.{ver.minor}" / "site-packages")
     assert path in result.out
+
+
+def test_uv_env_python_not_in_path(tox_project: ToxProjectCreator) -> None:
+    # Make sure there is no pythonX.Y in the search path
+    ver = sys.version_info
+    exe_ext = ".exe" if sys.platform == "win32" else ""
+    python_exe = f"python{ver.major}.{ver.minor}{exe_ext}"
+    env = dict(os.environ)
+    env["PATH"] = os.path.pathsep.join(
+        path for path in env["PATH"].split(os.path.pathsep) if not (pathlib.Path(path) / python_exe).is_file()
+    )
+
+    # Make sure the Python interpreter can find our Tox module
+    tox_spec = importlib.util.find_spec("tox")
+    assert tox_spec is not None
+    tox_lines = subprocess.check_output(
+        [sys.executable, "-c", "import tox; print(tox.__file__);"], encoding="UTF-8", env=env
+    ).splitlines()
+    assert tox_lines == [tox_spec.origin]
+
+    # Now use that Python interpreter to run Tox
+    project = tox_project({"tox.ini": "[testenv]\npackage=skip\ncommands=python -c 'print(\"{env_python}\")'"})
+    tox_ini = project.path / "tox.ini"
+    assert tox_ini.is_file()
+    subprocess.check_call([sys.executable, "-m", "tox", "-c", tox_ini], env=env)