✨ feat(py_info): add system_exe, a narrowed system executable

docs/changelog/127.feature.rst

@@ -0,0 +1,5 @@
+:attr:`~python_discovery.PythonInfo.system_exe` returns the executable of the system Python an interpreter is based on,
+falling back to :attr:`~python_discovery.PythonInfo.executable` the way
+:meth:`~python_discovery.PythonInfo.resolve_to_system` does. :attr:`~python_discovery.PythonInfo.system_executable` is
+``None`` until resolution runs, so every consumer had to narrow a value discovery has already settled - by
+:user:`gaborbernat`.

src/python_discovery/_py_info.py

@@ -167,6 +167,17 @@ def system_exec_prefix(self) -> str:
         """The exec prefix of the system Python this interpreter is based on."""
         return self.real_prefix or self.base_exec_prefix or self.exec_prefix
 
+    @property
+    def system_exe(self) -> str:
+        """
+        The executable of the system Python this interpreter is based on.
+
+        :attr:`system_executable` stays ``None`` until :meth:`resolve_to_system` walks the prefix chain, so reading it
+        forces every caller to narrow a value discovery has already settled. The fallback here matches what resolution
+        itself writes when a prefix links back to its own interpreter.
+        """
+        return self.system_executable or self.executable
+
     def __repr__(self) -> str:
         return "{}({!r})".format(
             self.__class__.__name__,

tests/py_info/test_py_info.py

@@ -511,3 +511,17 @@ def test_select_most_likely_prefers_machine_match(
         d.sysconfig_platform = plat
     result = PythonInfo._select_most_likely(discovered, target)
     assert result.sysconfig_platform == discovered_platforms[expected_idx]
+
+
+@pytest.mark.parametrize(
+    ("system_executable", "expected"),
+    [
+        pytest.param("/usr/bin/python3", "/usr/bin/python3", id="resolved"),
+        pytest.param(None, "/venv/bin/python", id="unresolved falls back to executable"),
+    ],
+)
+def test_py_info_system_exe(system_executable: str | None, expected: str) -> None:
+    info = copy.deepcopy(CURRENT)
+    info.executable = "/venv/bin/python"
+    info.system_executable = system_executable
+    assert info.system_exe == expected