Added a virtualenv backend for using uv

This also causes session.install to use uv instead of pip.

nox/_options.py

@@ -356,7 +356,7 @@ def _session_completer(
             " ``'virtualenv'`` by default but any of ``('virtualenv', 'conda', 'mamba',"
             " 'venv')`` are accepted."
         ),
-        choices=["none", "virtualenv", "conda", "mamba", "venv"],
+        choices=["none", "virtualenv", "conda", "mamba", "venv", "uv"],
     ),
     _option_set.Option(
         "force_venv_backend",
@@ -371,7 +371,7 @@ def _session_completer(
             " the default backend. Any of ``('virtualenv', 'conda', 'mamba', 'venv')``"
             " are accepted."
         ),
-        choices=["none", "virtualenv", "conda", "mamba", "venv"],
+        choices=["none", "virtualenv", "conda", "mamba", "venv", "uv"],
     ),
     _option_set.Option(
         "no_venv",

nox/sessions.py

@@ -632,7 +632,7 @@ def install(self, *args: str, **kwargs: Any) -> None:
         if "silent" not in kwargs:
             kwargs["silent"] = True
 
-        if getattr(venv, "venv_prog", "") == "uv":
+        if isinstance(venv, VirtualEnv) and venv.venv_backend == "uv":
             self._run("uv", "pip", "install", *args, external="error", **kwargs)
         else:
             self._run(
@@ -758,7 +758,7 @@ def _create_venv(self) -> None:
                 self.envdir,
                 interpreter=self.func.python,  # type: ignore[arg-type]
                 reuse_existing=reuse_existing,
-                venv=backend or "virtualenv",
+                venv_backend=backend or "virtualenv",
                 venv_params=self.func.venv_params,
             )
         elif backend in {"conda", "mamba"}:

nox/virtualenv.py

@@ -320,15 +320,15 @@ def __init__(
         interpreter: str | None = None,
         reuse_existing: bool = False,
         *,
-        venv: str = "virtualenv",
+        venv_backend: str = "virtualenv",
         venv_params: Any = None,
     ):
         self.location_name = location
         self.location = os.path.abspath(location)
         self.interpreter = interpreter
         self._resolved: None | str | InterpreterNotFound = None
         self.reuse_existing = reuse_existing
-        self.venv_prog = venv
+        self.venv_backend = venv_backend
         self.venv_params = venv_params or []
         super().__init__(env={"VIRTUAL_ENV": self.location})
 
@@ -360,7 +360,11 @@ def _check_reused_environment_type(self) -> bool:
                 old_env = (
                     "virtualenv" if any(pattern.match(line) for line in fp) else "venv"
                 )
-        return old_env == self.venv_prog
+        # We can't distinguish a uv env from a venv env, so just treat them
+        # the same.
+        return old_env == self.venv_backend or (
+            {old_env, self.venv_backend} == {"uv", "venv"}
+        )
 
     def _check_reused_environment_interpreter(self) -> bool:
         """Check if reused environment interpreter is the same."""
@@ -475,11 +479,11 @@ def create(self) -> bool:
 
             return False
 
-        if self.venv_prog == "virtualenv":
+        if self.venv_backend == "virtualenv":
             cmd = [sys.executable, "-m", "virtualenv", self.location]
             if self.interpreter:
                 cmd.extend(["-p", self._resolved_interpreter])
-        elif self.venv_prog == "uv":
+        elif self.venv_backend == "uv":
             cmd = [
                 "uv",
                 "virtualenv",
@@ -494,7 +498,7 @@ def create(self) -> bool:
         resolved_interpreter_name = os.path.basename(self._resolved_interpreter)
 
         logger.info(
-            f"Creating virtual environment ({self.venv_prog}) using"
+            f"Creating virtual environment ({self.venv_backend}) using"
             f" {resolved_interpreter_name} in {self.location_name}"
         )
         nox.command.run(cmd, silent=True, log=nox.options.verbose or False)

pyproject.toml

@@ -53,7 +53,7 @@ tox_to_nox = [
   "tox<4",
 ]
 uv = [
-  "uv"
+  "uv",
 ]
 [project.urls]
 bug-tracker = "https://github.com/wntrblm/nox/issues"

tests/test_sessions.py

@@ -79,6 +79,7 @@ def make_session_and_runner(self):
         runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
         runner.venv.env = {}
         runner.venv.bin_paths = ["/no/bin/for/you"]
+        runner.venv.venv_backend = "venv"
         return nox.sessions.Session(runner=runner), runner
 
     def test_create_tmp(self):
@@ -629,6 +630,7 @@ def test_install(self):
         )
         runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
         runner.venv.env = {}
+        runner.venv.venv_backend = "venv"
 
         class SessionNoSlots(nox.sessions.Session):
             pass
@@ -658,6 +660,7 @@ def test_install_non_default_kwargs(self):
         )
         runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
         runner.venv.env = {}
+        runner.venv.venv_backend = "venv"
 
         class SessionNoSlots(nox.sessions.Session):
             pass
@@ -794,6 +797,35 @@ def test_session_venv_reused_with_no_install(self, no_install, reused, run_calle
 
         assert run.called is run_called
 
+    def test_install_uv(self):
+        runner = nox.sessions.SessionRunner(
+            name="test",
+            signatures=["test"],
+            func=mock.sentinel.func,
+            global_config=_options.options.namespace(posargs=[]),
+            manifest=mock.create_autospec(nox.manifest.Manifest),
+        )
+        runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
+        runner.venv.env = {}
+        runner.venv.venv_backend = "uv"
+
+        class SessionNoSlots(nox.sessions.Session):
+            pass
+
+        session = SessionNoSlots(runner=runner)
+
+        with mock.patch.object(session, "_run", autospec=True) as run:
+            session.install("requests", "urllib3", silent=False)
+            run.assert_called_once_with(
+                "uv",
+                "pip",
+                "install",
+                "requests",
+                "urllib3",
+                silent=False,
+                external=True,
+            )
+
     def test___slots__(self):
         session, _ = self.make_session_and_runner()
         with pytest.raises(AttributeError):

tests/test_virtualenv.py

@@ -31,6 +31,7 @@
 
 IS_WINDOWS = nox.virtualenv._SYSTEM == "Windows"
 HAS_CONDA = shutil.which("conda") is not None
+HAS_UV = shutil.which("uv") is not None
 RAISE_ERROR = "RAISE_ERROR"
 VIRTUALENV_VERSION = virtualenv.__version__
 
@@ -240,6 +241,17 @@ def test_condaenv_detection(make_conda):
     assert path_regex.search(output).group("env_dir") == dir_.strpath
 
 
+@pytest.mark.skipif(not HAS_UV, reason="Missing uv command.")
+def test_uv_creation(make_one):
+    venv, _ = make_one(venv_backend="uv")
+    assert venv.location
+    assert venv.interpreter is None
+    assert venv.reuse_existing is False
+    assert venv.venv_backend == "uv"
+
+    venv.create()
+
+
 def test_constructor_defaults(make_one):
     venv, _ = make_one()
     assert venv.location
@@ -417,7 +429,7 @@ def test_create_reuse_stale_venv_environment(make_one):
 
 @enable_staleness_check
 def test_create_reuse_stale_virtualenv_environment(make_one):
-    venv, location = make_one(reuse_existing=True, venv=True)
+    venv, location = make_one(reuse_existing=True, venv_backend="venv")
     venv.create()
 
     # Drop a virtualenv-style pyvenv.cfg into the environment.
@@ -442,7 +454,7 @@ def test_create_reuse_stale_virtualenv_environment(make_one):
 
 @enable_staleness_check
 def test_create_reuse_venv_environment(make_one):
-    venv, location = make_one(reuse_existing=True, venv="venv")
+    venv, location = make_one(reuse_existing=True, venv_backend="venv")
     venv.create()
 
     # Place a spurious occurrence of "virtualenv" in the pyvenv.cfg.
@@ -516,7 +528,7 @@ def test_create_reuse_python2_environment(make_one):
 
 
 def test_create_venv_backend(make_one):
-    venv, dir_ = make_one(venv=True)
+    venv, dir_ = make_one(venv_backend="venv")
     venv.create()