Revert to supporting simple Python factors

Instead of parsing factors with virtualenv's
'PythonSpec.from_string_spec', use our own regex to support the simple
factors that have been supported since tox v1.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: #2657
Fixes: #2848

docs/changelog/2848.bugfix.rst

@@ -0,0 +1,5 @@
+tox has reverted support for complex Python factors such as ``py3.10``, ``cpython3.10.1`` or ``pypy3-64``. This was
+initially supported with the release of tox 4.0 but has proven complicated to support. Instead, the simple factors
+supported by tox 3 such as ``py310`` or ``pypy3`` should be used. Users who wish to specify more specific Python version
+information should configure the ``base_python`` setting in the relevant ``[testenv]`` section(s) in ``tox.ini`` - by
+:user:`stephenfin`.

src/tox/tox_env/python/api.py

@@ -4,19 +4,21 @@
 from __future__ import annotations
 
 import logging
+import re
 import sys
 from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import Any, List, NamedTuple, cast
 
-from packaging.tags import INTERPRETER_SHORT_NAMES
 from virtualenv.discovery.py_spec import PythonSpec
 
 from tox.config.main import Config
 from tox.tox_env.api import ToxEnv, ToxEnvCreateArgs
 from tox.tox_env.errors import Fail, Recreate, Skip
 from tox.util.ci import is_ci
 
+PY_FACTORS_RE = re.compile("^(?!py$)(?P<impl>py|pypy|jython)(?P<version>[2-9][0-9]?[0-9]?)?$")
+
 
 class VersionInfo(NamedTuple):
     major: int
@@ -131,9 +133,8 @@ def default_base_python(self, conf: Config, env_name: str | None) -> list[str]:
     def extract_base_python(cls, env_name: str) -> str | None:
         candidates: list[str] = []
         for factor in env_name.split("-"):
-            spec = PythonSpec.from_string_spec(factor)
-            impl = spec.implementation or "python"
-            if impl.lower() in INTERPRETER_SHORT_NAMES and env_name is not None and spec.path is None:
+            match = PY_FACTORS_RE.match(factor)
+            if match:
                 candidates.append(factor)
         if candidates:
             if len(candidates) > 1:

tests/tox_env/python/test_python_api.py

@@ -89,16 +89,13 @@ def test_base_python_env_no_conflict(env: str, base_python: list[str], ignore_co
 @pytest.mark.parametrize(
     ("env", "base_python", "expected", "conflict"),
     [
-        ("cpython", ["pypy"], "cpython", ["pypy"]),
         ("pypy", ["cpython"], "pypy", ["cpython"]),
         ("pypy2", ["pypy3"], "pypy2", ["pypy3"]),
         ("py3", ["py2"], "py3", ["py2"]),
         ("py38", ["py39"], "py38", ["py39"]),
         ("py38", ["py38", "py39"], "py38", ["py39"]),
         ("py38", ["python3"], "py38", ["python3"]),
         ("py310", ["py38", "py39"], "py310", ["py38", "py39"]),
-        ("py3.11.1", ["py3.11.2"], "py3.11.1", ["py3.11.2"]),
-        ("py3-64", ["py3-32"], "py3-64", ["py3-32"]),
         ("py310-magic", ["py39"], "py310", ["py39"]),
     ],
     ids=lambda a: "|".join(a) if isinstance(a, list) else str(a),
@@ -110,9 +107,6 @@ def test_base_python_env_conflict(
     conflict: list[str],
     ignore_conflict: bool,
 ) -> None:
-    if env == "py3-64":
-        raise pytest.skip("bug #2657")
-
     if ignore_conflict:
         result = Python._validate_base_python(env, base_python, ignore_conflict)
         assert result == [expected]

tests/tox_env/python/virtual_env/test_virtualenv_api.py

@@ -33,10 +33,10 @@ def virtualenv_opt(monkeypatch: MonkeyPatch, mocker: MockerFixture) -> VirtualEn
 
 def test_virtualenv_default_settings(tox_project: ToxProjectCreator, virtualenv_opt: VirtualEnvOptions) -> None:
     proj = tox_project({"tox.ini": "[testenv]\npackage=skip"})
-    result = proj.run("r", "-e", "py", "--discover", sys.executable, str(proj.path / "a"))
+    result = proj.run("r", "-e", "py3", "--discover", sys.executable, str(proj.path / "a"))
     result.assert_success()
 
-    conf = result.env_conf("py")
+    conf = result.env_conf("py3")
     assert conf["system_site_packages"] is False
     assert conf["always_copy"] is False
     assert conf["download"] is False
@@ -46,7 +46,7 @@ def test_virtualenv_default_settings(tox_project: ToxProjectCreator, virtualenv_
     assert virtualenv_opt.download is False
     assert virtualenv_opt.copies is False
     assert virtualenv_opt.no_periodic_update is True
-    assert virtualenv_opt.python == ["py"]
+    assert virtualenv_opt.python == ["py3"]
     assert virtualenv_opt.try_first_with == [str(sys.executable), str(proj.path / "a")]
 
 
@@ -60,10 +60,10 @@ def test_virtualenv_flipped_settings(
     )
     monkeypatch.setenv("VIRTUALENV_CLEAR", "0")
 
-    result = proj.run("r", "-e", "py")
+    result = proj.run("r", "-e", "py3")
     result.assert_success()
 
-    conf = result.env_conf("py")
+    conf = result.env_conf("py3")
     assert conf["system_site_packages"] is True
     assert conf["always_copy"] is True
     assert conf["download"] is True
@@ -72,7 +72,7 @@ def test_virtualenv_flipped_settings(
     assert virtualenv_opt.system_site is True
     assert virtualenv_opt.download is True
     assert virtualenv_opt.copies is True
-    assert virtualenv_opt.python == ["py"]
+    assert virtualenv_opt.python == ["py3"]
 
 
 def test_virtualenv_env_ignored_if_set(