tests: monkeypatch lower down for new test

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

nox/sessions.py

@@ -500,8 +500,12 @@ def _run(
             return self._run_func(args[0], args[1:], kwargs)  # type: ignore[unreachable]
 
         # Using `"uv"` when `uv` is the backend is guaranteed to work, even if it was co-installed with nox.
-        if self.virtualenv.venv_backend == "uv" and args[0] == "uv" and nox.virtualenv.UV != "uv":
-            args = (nox.virualenv.UV, *args[1:])
+        if (
+            self.virtualenv.venv_backend == "uv"
+            and args[0] == "uv"
+            and nox.virtualenv.UV != "uv"
+        ):
+            args = (nox.virtualenv.UV, *args[1:])
 
         # Combine the env argument with our virtualenv's env vars.
         if include_outer_env:

tests/test_sessions.py

@@ -823,8 +823,7 @@ def test_session_venv_reused_with_no_install(self, no_install, reused, run_calle
 
         assert run.called is run_called
 
-    @pytest.mark.parametrize("uv", [None, "/some/uv"])
-    def test_install_uv(self, uv, monkeypatch):
+    def test_install_uv(self):
         runner = nox.sessions.SessionRunner(
             name="test",
             signatures=["test"],
@@ -841,13 +840,10 @@ class SessionNoSlots(nox.sessions.Session):
 
         session = SessionNoSlots(runner=runner)
 
-        if uv is not None:
-            monkeypatch.setattr(nox.virtualenv, "UV", uv)
-
         with mock.patch.object(session, "_run", autospec=True) as run:
             session.install("requests", "urllib3", silent=False)
             run.assert_called_once_with(
-                nox.virtualenv.UV,
+                "uv",
                 "pip",
                 "install",
                 "requests",
@@ -856,6 +852,39 @@ class SessionNoSlots(nox.sessions.Session):
                 external="error",
             )
 
+    def test_install_uv_command(self, monkeypatch):
+        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)
+
+        monkeypatch.setattr(nox.virtualenv, "UV", "/some/uv")
+        monkeypatch.setattr(shutil, "which", lambda x: None)
+
+        with mock.patch.object(nox.command, "run", autospec=True) as run:
+            session.install("requests", "urllib3", silent=False)
+            run.assert_called_once()
+
+            ((call_args,), _) = run.call_args
+            assert call_args == (
+                "/some/uv",
+                "pip",
+                "install",
+                "requests",
+                "urllib3",
+            )
+
     def test___slots__(self):
         session, _ = self.make_session_and_runner()
         with pytest.raises(AttributeError):