Fixup the spec string for sys.executable (#3327)

* Fixup the spec string for sys.executable

The previous spec string was:

    namespace(name='cpython', cache_tag='cpython-313', version=sys.version_info(major=3, minor=13, micro=0, releaselevel='candidate', serial=1), hexversion=51183809, _multiarch='x86_64-linux-gnu')313-True

When it was supposed to be:

    cpython313-64

Fixes https://github.com/tox-dev/tox/pull/3325/files#r1718230254
Fixes https://github.com/tox-dev/tox/pull/3325/files#r1718246292

Adds tests for a new method.

* Make the new method private

* Silence the linter, this is a test for a private method

The error was:

    tests/tox_env/python/test_python_api.py:314:12: SLF001 Private member accessed: `_python_spec_for_sys_executable`
        |
    312 |     mocker.patch.object(sys, "implementation", implementation)
    313 |     mocker.patch.object(sys, "maxsize", 2**arch // 2 - 1)
    314 |     spec = Python._python_spec_for_sys_executable()
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ SLF001
    315 |     assert spec.implementation == impl
    316 |     assert spec.major == major
        |

    Found 1 error.

* Add a changelog fragment

Author: hroncok

docs/changelog/3327.bugfix.rst

@@ -0,0 +1,2 @@
+Fix and test the string spec for the ``sys.executable`` interpreter (introduced in :pull:`3325`)
+- by :user:`hroncok`

src/tox/tox_env/python/api.py

@@ -157,6 +157,14 @@ def extract_base_python(cls, env_name: str) -> str | None:
             return next(iter(candidates))
         return None
 
+    @classmethod
+    def _python_spec_for_sys_executable(cls) -> PythonSpec:
+        implementation = sys.implementation.name
+        version = sys.version_info
+        bits = "64" if sys.maxsize > 2**32 else "32"
+        string_spec = f"{implementation}{version.major}{version.minor}-{bits}"
+        return PythonSpec.from_string_spec(string_spec)
+
     @classmethod
     def _validate_base_python(
         cls,
@@ -172,8 +180,7 @@ def _validate_base_python(
                 if spec_base.path is not None:
                     path = Path(spec_base.path).absolute()
                     if str(spec_base.path) == sys.executable:
-                        ver, is_64 = sys.version_info, sys.maxsize != 2**32
-                        spec_base = PythonSpec.from_string_spec(f"{sys.implementation}{ver.major}{ver.minor}-{is_64}")
+                        spec_base = cls._python_spec_for_sys_executable()
                     else:
                         spec_base = cls.python_spec_for_path(path)
                 if any(

tests/tox_env/python/test_python_api.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import sys
+from types import SimpleNamespace
 from typing import TYPE_CHECKING, Callable
 from unittest.mock import patch
 
@@ -289,3 +290,29 @@ def test_usedevelop_with_nonexistent_basepython(tox_project: ToxProjectCreator)
     project = tox_project({"tox.ini": ini})
     result = project.run()
     assert result.code == 0
+
+
+@pytest.mark.parametrize(
+    ("impl", "major", "minor", "arch"),
+    [
+        ("cpython", 3, 12, 64),
+        ("pypy", 3, 9, 32),
+    ],
+)
+def test_python_spec_for_sys_executable(impl: str, major: int, minor: int, arch: int, mocker: MockerFixture) -> None:
+    version_info = SimpleNamespace(major=major, minor=minor, micro=5, releaselevel="final", serial=0)
+    implementation = SimpleNamespace(
+        name=impl,
+        cache_tag=f"{impl}-{major}{minor}",
+        version=version_info,
+        hexversion=...,
+        _multiarch=...,
+    )
+    mocker.patch.object(sys, "version_info", version_info)
+    mocker.patch.object(sys, "implementation", implementation)
+    mocker.patch.object(sys, "maxsize", 2**arch // 2 - 1)
+    spec = Python._python_spec_for_sys_executable()  # noqa: SLF001
+    assert spec.implementation == impl
+    assert spec.major == major
+    assert spec.minor == minor
+    assert spec.architecture == arch