Show host python under core section as constant (#2644)

Author: gaborbernat

docs/changelog/2630.bugfix.rst

@@ -0,0 +1,2 @@
+The core tox configuration now contains ``host_python`` key showing the host python executable path -
+by :user:`gaborbernat`.

src/tox/config/sets.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import sys
 from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Callable, Iterator, Mapping, Sequence, TypeVar, cast
@@ -202,6 +203,7 @@ def work_dir_builder(conf: Config, env_name: str | None) -> Path:  # noqa: U100
             default=lambda conf, _: cast(Path, self["work_dir"]) / ".tmp",  # noqa: U100, U101
             desc="a folder for temporary files (is not cleaned at start)",
         )
+        self.add_constant("host_python", "the host python executable path", sys.executable)
 
     def _on_duplicate_conf(self, key: str, definition: ConfigDefinition[V]) -> None:  # noqa: U100
         pass  # core definitions may be defined multiple times as long as all their options match, first defined wins

tests/session/cmd/test_show_config.py

@@ -219,3 +219,10 @@ def test_show_config_timeout_custom(tox_project: ToxProjectCreator) -> None:
 def test_show_config_help(tox_project: ToxProjectCreator) -> None:
     outcome = tox_project({"tox.ini": ""}).run("c", "-h")
     outcome.assert_success()
+
+
+def test_show_config_core_host_python(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({"tox.ini": ""})
+    outcome = project.run("c", "--core", "-e", "py", "-k", "host_python")
+    outcome.assert_success()
+    assert f"host_python = {sys.executable}" in outcome.out