Fix UV_PYTHON_PREFERENCE reading breaks tox when env_site_packages_dir is in set_env

.pre-commit-config.yaml

@@ -10,12 +10,12 @@ repos:
       - id: check-github-workflows
         args: ["--verbose"]
   - repo: https://github.com/codespell-project/codespell
-    rev: v2.3.0
+    rev: v2.4.0
     hooks:
       - id: codespell
         additional_dependencies: ["tomli>=2.2.1"]
   - repo: https://github.com/tox-dev/tox-ini-fmt
-    rev: "1.4.1"
+    rev: "1.5.0"
     hooks:
       - id: tox-ini-fmt
         args: ["-p", "fix"]

pyproject.toml

@@ -43,7 +43,7 @@ dependencies = [
   "packaging>=24.2",
   "tox>=4.24.1,<5",
   "typing-extensions>=4.12.2; python_version<'3.10'",
-  "uv>=0.5.21,<1",
+  "uv>=0.5.22,<1",
 ]
 urls.Changelog = "https://github.com/tox-dev/tox-uv/releases"
 urls.Documentation = "https://github.com/tox-dev/tox-uv#tox-uv"
@@ -69,7 +69,7 @@ test = [
 ]
 type = [ "mypy==1.14.1", { include-group = "test" } ]
 lint = [ "pre-commit-uv>=4.1.4" ]
-pkg-meta = [ "check-wheel-contents>=0.6.1", "twine>=6.1", "uv>=0.5.21" ]
+pkg-meta = [ "check-wheel-contents>=0.6.1", "twine>=6.1", "uv>=0.5.22" ]
 
 [tool.hatch]
 build.hooks.vcs.version-file = "src/tox_uv/version.py"

src/tox_uv/_venv.py

@@ -4,6 +4,7 @@
 
 import json
 import logging
+import os
 import sys
 from abc import ABC
 from functools import cached_property
@@ -73,7 +74,9 @@ def register_config(self) -> None:
         self.conf.add_config(
             keys=["uv_python_preference"],
             of_type=cast("Type[Optional[PythonPreference]]", Optional[PythonPreference]),  # noqa: UP006
-            default=lambda conf, name: self.environment_variables.get("UV_PYTHON_PREFERENCE", "system"),  # noqa: ARG005
+            # use os.environ here instead of self.environment_variables as this value is needed to create the virtual
+            # environment, if environment variables use env_site_packages_dir we would run into a chicken-egg problem.
+            default=lambda conf, name: os.environ.get("UV_PYTHON_PREFERENCE", "system"),  # noqa: ARG005
             desc=(
                 "Whether to prefer using Python installations that are already"
                 " present on the system, or those that are downloaded and"
@@ -240,10 +243,10 @@ def env_python(self) -> Path:
     def env_site_package_dir(self) -> Path:
         if sys.platform == "win32":  # pragma: win32 cover
             return self.venv_dir / "Lib" / "site-packages"
-        else:  # pragma: win32 no cover # noqa: RET505
-            py = self._py_info
-            impl = "pypy" if py.implementation == "pypy" else "python"
-            return self.venv_dir / "lib" / f"{impl}{py.version_dot}" / "site-packages"
+        # pragma: win32 no cover
+        py = self._py_info
+        impl = "pypy" if py.implementation == "pypy" else "python"
+        return self.venv_dir / "lib" / f"{impl}{py.version_dot}" / "site-packages"
 
     def env_version_spec(self) -> str:
         base = self.base_python.version_info

tests/test_tox_uv_venv.py

@@ -71,6 +71,18 @@ def test_uv_venv_preference_override_via_env_var(
     assert got == "only-managed"
 
 
+def test_uv_venv_preference_override_via_env_var_and_set_env_depends_on_py(
+    tox_project: ToxProjectCreator, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    project = tox_project({"tox.ini": "[testenv]\nset_env=A={env_site_packages_dir}"})
+    monkeypatch.setenv("UV_PYTHON_PREFERENCE", "only-managed")
+
+    result = project.run("c", "-k", "set_env")
+    result.assert_success()
+
+    assert str(project.path) in result.out
+
+
 def test_uv_venv_spec(tox_project: ToxProjectCreator) -> None:
     ver = sys.version_info
     project = tox_project({"tox.ini": f"[testenv]\npackage=skip\nbase_python={ver.major}.{ver.minor}"})

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 requires =
     tox>=4.24.1
-    tox-uv>=1.19.1
+    tox-uv>=1.20
 env_list =
     fix
     3.13