feat: add venv_backend property

Signed-off-by: Henry Schreiner <[email protected]>

nox/sessions.py

@@ -193,6 +193,11 @@ 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."""
+ return self._runner.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(