fix: ensure 'uv' always works in a uv venv

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

nox/sessions.py

@@ -499,6 +499,15 @@ def _run(
         if callable(args[0]):
             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 (
+            isinstance(self.virtualenv, VirtualEnv)
+            and self.virtualenv.venv_backend == "uv"
+            and args[0] == "uv"
+            and UV != "uv"
+        ):
+            args = (UV, *args[1:])
+
         # Combine the env argument with our virtualenv's env vars.
         if include_outer_env:
             overlay_env = env
@@ -670,7 +679,7 @@ def install(self, *args: str, **kwargs: Any) -> None:
             kwargs["silent"] = True
 
         if isinstance(venv, VirtualEnv) and venv.venv_backend == "uv":
-            self._run(UV, "pip", "install", *args, external="error", **kwargs)
+            self._run("uv", "pip", "install", *args, external="error", **kwargs)
         else:
             self._run(
                 "python", "-m", "pip", "install", *args, external="error", **kwargs

nox/virtualenv.py

@@ -47,9 +47,8 @@ def find_uv() -> tuple[bool, str]:
         return True, find_uv_bin()
 
     # Fall back to PATH.
-    uv = shutil.which("uv")
-    if uv is not None:
-        return True, uv
+    if shutil.which("uv") is not None:
+        return True, "uv"
 
     return False, "uv"
 

tests/test_sessions.py

@@ -840,10 +840,11 @@ class SessionNoSlots(nox.sessions.Session):
 
         session = SessionNoSlots(runner=runner)
 
+        uv = "uv" if shutil.which("uv") is not None else nox.virtualenv.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",

tests/test_virtualenv.py

@@ -605,7 +605,7 @@ def test_create_reuse_uv_environment(make_one):
     ["which_result", "find_uv_bin_result", "expected"],
     [
         ("/usr/bin/uv", UV_IN_PIPX_VENV,   (True,  UV_IN_PIPX_VENV)),
-        ("/usr/bin/uv", FileNotFoundError, (True,  "/usr/bin/uv")),
+        ("/usr/bin/uv", FileNotFoundError, (True,  "uv")),
         (None,          UV_IN_PIPX_VENV,   (True,  UV_IN_PIPX_VENV)),
         (None,          FileNotFoundError, (False, "uv")),
     ],