fix: ensure 'uv' always works in a uv venv (#818)

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

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

* fix: always use simplest form if on PATH

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

* tests: verify correction works

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

* tests: monkeypatch lower down for new test

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

* fix: handle corner case of user instalilng uv into a uv environment

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

* tests: add tests for installed UV

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

---------

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

nox/sessions.py

@@ -21,6 +21,7 @@
 import os
 import pathlib
 import re
+import shutil
 import subprocess
 import sys
 import unicodedata
@@ -43,7 +44,7 @@
 from nox._decorators import Func
 from nox.logger import logger
 from nox.popen import DEFAULT_INTERRUPT_TIMEOUT, DEFAULT_TERMINATE_TIMEOUT
-from nox.virtualenv import UV, CondaEnv, PassthroughEnv, ProcessEnv, VirtualEnv
+from nox.virtualenv import CondaEnv, PassthroughEnv, ProcessEnv, VirtualEnv
 
 if TYPE_CHECKING:
     from typing import IO
@@ -556,6 +557,15 @@ def _run(
         if callable(args[0]):
             return self._run_func(args[0], args[1:])  # 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"
+            and shutil.which("uv", path=self.bin) is None  # Session uv takes priority
+        ):
+            args = (nox.virtualenv.UV, *args[1:])
+
         # Combine the env argument with our virtualenv's env vars.
         if include_outer_env:
             overlay_env = env or {}
@@ -769,7 +779,7 @@ def install(
             silent = True
 
         if isinstance(venv, VirtualEnv) and venv.venv_backend == "uv":
-            cmd = [UV, "pip", "install"]
+            cmd = ["uv", "pip", "install"]
         else:
             cmd = ["python", "-m", "pip", "install"]
         self._run(

nox/virtualenv.py

@@ -24,6 +24,7 @@
 import subprocess
 import sys
 from collections.abc import Callable, Mapping
+from pathlib import Path
 from socket import gethostbyname
 from typing import Any, ClassVar
 
@@ -45,18 +46,23 @@
 
 
 def find_uv() -> tuple[bool, str]:
+    uv_on_path = shutil.which("uv")
+
     # Look for uv in Nox's environment, to handle `pipx install nox[uv]`.
     with contextlib.suppress(ImportError, FileNotFoundError):
         from uv import find_uv_bin
 
-        return True, find_uv_bin()
+        uv_bin = find_uv_bin()
 
-    # Fall back to PATH.
-    uv = shutil.which("uv")
-    if uv is not None:
-        return True, uv
+        # If the returned value is the same as calling "uv" already, don't
+        # expand (simpler logging)
+        if uv_on_path and Path(uv_bin).samefile(uv_on_path):
+            return True, "uv"
 
-    return False, "uv"
+        return True, uv_bin
+
+    # Fall back to PATH.
+    return uv_on_path is not None, "uv"
 
 
 HAS_UV, UV = find_uv()

tests/test_sessions.py

@@ -869,14 +869,65 @@ class SessionNoSlots(nox.sessions.Session):
         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",
                 "urllib3",
                 **_run_with_defaults(silent=False, 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, path=None: 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",
+            )
+
+        # user installs uv in the session venv
+        monkeypatch.setattr(
+            shutil, "which", lambda x, path="": path + "/uv" if x == "uv" else 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 == (
+                "uv",
+                "pip",
+                "install",
+                "requests",
+                "urllib3",
+            )
+
     def test___slots__(self):
         session, _ = self.make_session_and_runner()
         with pytest.raises(AttributeError):

tests/test_virtualenv.py

@@ -21,6 +21,7 @@
 import subprocess
 import sys
 import types
+from pathlib import Path
 from textwrap import dedent
 from typing import NamedTuple
 from unittest import mock
@@ -602,26 +603,27 @@ def test_create_reuse_uv_environment(make_one):
 
 
 @pytest.mark.parametrize(
-    ["which_result", "find_uv_bin_result", "expected"],
+    ["which_result", "find_uv_bin_result", "found", "path"],
     [
-        ("/usr/bin/uv", UV_IN_PIPX_VENV,   (True,  UV_IN_PIPX_VENV)),
-        ("/usr/bin/uv", FileNotFoundError, (True,  "/usr/bin/uv")),
-        (None,          UV_IN_PIPX_VENV,   (True,  UV_IN_PIPX_VENV)),
-        (None,          FileNotFoundError, (False, "uv")),
+        ("/usr/bin/uv", UV_IN_PIPX_VENV, True, UV_IN_PIPX_VENV),
+        ("/usr/bin/uv", FileNotFoundError, True, "uv"),
+        (None, UV_IN_PIPX_VENV, True, UV_IN_PIPX_VENV),
+        (None, FileNotFoundError, False, "uv"),
     ],
-)  # fmt: skip
-def test_find_uv(monkeypatch, which_result, find_uv_bin_result, expected):
+)
+def test_find_uv(monkeypatch, which_result, find_uv_bin_result, found, path):
     def find_uv_bin():
         if find_uv_bin_result is FileNotFoundError:
             raise FileNotFoundError
         return find_uv_bin_result
 
     monkeypatch.setattr(shutil, "which", lambda _: which_result)
+    monkeypatch.setattr(Path, "samefile", lambda a, b: a == b)
     monkeypatch.setitem(
         sys.modules, "uv", types.SimpleNamespace(find_uv_bin=find_uv_bin)
     )
 
-    assert nox.virtualenv.find_uv() == expected
+    assert nox.virtualenv.find_uv() == (found, path)
 
 
 def test_create_reuse_venv_environment(make_one, monkeypatch):