fix(tox_env.python): do not process absolute paths to interpreter as PythonSpec

docs/changelog/3191.bugfix.rst

@@ -0,0 +1 @@
+``base_python`` now accepts absolute paths to interpreter executable - by :user:`paveldikov`.

src/tox/tox_env/python/api.py

@@ -6,6 +6,7 @@
 import re
 import sys
 from abc import ABC, abstractmethod
+from pathlib import Path
 from typing import TYPE_CHECKING, Any, List, NamedTuple, cast
 
 from virtualenv.discovery.py_spec import PythonSpec
@@ -14,7 +15,6 @@
 from tox.tox_env.errors import Fail, Recreate, Skip
 
 if TYPE_CHECKING:
-    from pathlib import Path
 
     from tox.config.main import Config
 
@@ -169,6 +169,8 @@ def _validate_base_python(
         if env_base_python is not None:
             spec_name = PythonSpec.from_string_spec(env_base_python)
             for base_python in base_pythons:
+                if Path(base_python).is_absolute():
+                    return [base_python]
                 spec_base = PythonSpec.from_string_spec(base_python)
                 if any(
                     getattr(spec_base, key) != getattr(spec_name, key)

tests/tox_env/python/test_python_api.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 import sys
 from typing import TYPE_CHECKING, Callable
 from unittest.mock import patch
@@ -125,6 +126,24 @@ def test_base_python_env_no_conflict(env: str, base_python: list[str], ignore_co
     assert result is base_python
 
 
+@pytest.mark.parametrize(
+    ("env", "base_python", "platform"),
+    [
+        ("py312-unix", ["/opt/python312/bin/python"], "posix"),
+        ("py312-win", [r"C:\Program Files\Python312\python.exe"], "nt"),
+        ("py311-win", [r"\\a\python311\python.exe"], "nt"),
+        ("py310-win", [r"\\?\UNC\a\python310\python.exe"], "nt"),
+        ("py310", ["//a/python310/bin/python"], None),
+    ],
+    ids=lambda a: "|".join(a) if isinstance(a, list) else str(a),
+)
+def test_base_python_absolute(env: str, base_python: list[str], platform: str | None) -> None:
+    if platform and platform != os.name:
+        pytest.skip(f"Not applicable to this platform. ({platform} != {os.name})")
+    result = Python._validate_base_python(env, base_python, False)  # noqa: SLF001
+    assert result == base_python
+
+
 @pytest.mark.parametrize("ignore_conflict", [True, False])
 @pytest.mark.parametrize(
     ("env", "base_python", "expected", "conflict"),