feat: add venv_backend property

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

nox/sessions.py

@@ -193,6 +193,14 @@ def virtualenv(self) -> ProcessEnv:
             raise ValueError("A virtualenv has not been created for this session")
         return venv
 
+    @property
+    def venv_backend(self) -> str:
+        """The venv_backend selected."""
+        venv = self._runner.venv
+        if venv is None:
+            return "none"
+        return venv.venv_backend
+
     @property
     def python(self) -> str | Sequence[str] | bool | None:
         """The python version passed into ``@nox.session``."""

nox/virtualenv.py

@@ -93,6 +93,13 @@ def create(self) -> bool:
         Returns True if the environment is new, and False if it was reused.
         """
 
+    @property
+    @abc.abstractmethod
+    def venv_backend(self) -> str:
+        """
+        Returns the string used to select this environment.
+        """
+
 
 def locate_via_py(version: str) -> str | None:
     """Find the Python executable using the Windows Launcher.
@@ -180,6 +187,10 @@ def create(self) -> bool:
         False since it's always reused."""
         return False
 
+    @property
+    def venv_backend(self) -> str:
+        return "none"
+
 
 class CondaEnv(ProcessEnv):
     """Conda environment management class.
@@ -303,6 +314,10 @@ def is_offline() -> bool:
         except BaseException:  # pragma: no cover
             return True
 
+    @property
+    def venv_backend(self) -> str:
+        return self.conda_cmd
+
 
 class VirtualEnv(ProcessEnv):
     """Virtualenv management class.
@@ -341,7 +356,7 @@ def __init__(
         self.interpreter = interpreter
         self._resolved: None | str | InterpreterNotFound = None
         self.reuse_existing = reuse_existing
-        self.venv_backend = venv_backend
+        self._venv_backend = venv_backend
         self.venv_params = venv_params or []
         if venv_backend not in {"virtualenv", "venv", "uv"}:
             msg = f"venv_backend {venv_backend} not recognized"
@@ -544,6 +559,10 @@ def create(self) -> bool:
 
         return True
 
+    @property
+    def venv_backend(self) -> str:
+        return self._venv_backend
+
 
 ALL_VENVS: dict[str, Callable[..., ProcessEnv]] = {
     "conda": functools.partial(CondaEnv, conda_cmd="conda"),

tests/test_sessions.py

@@ -641,6 +641,8 @@ class SessionNoSlots(nox.sessions.Session):
 
         session = SessionNoSlots(runner=runner)
 
+        assert session.venv_backend == "venv"
+
         with mock.patch.object(session, "_run", autospec=True) as run:
             session.install("requests", "urllib3")
             run.assert_called_once_with(