fix(env): ensure all system site paths are used

src/poetry/utils/env/__init__.py

@@ -21,7 +21,6 @@
 from poetry.utils.env.script_strings import GET_ENV_PATH_ONELINER
 from poetry.utils.env.script_strings import GET_ENVIRONMENT_INFO
 from poetry.utils.env.script_strings import GET_PATHS
-from poetry.utils.env.script_strings import GET_PATHS_FOR_GENERIC_ENVS
 from poetry.utils.env.script_strings import GET_PYTHON_VERSION_ONELINER
 from poetry.utils.env.script_strings import GET_SYS_PATH
 from poetry.utils.env.site_packages import SitePackages
@@ -97,7 +96,6 @@ def build_environment(
     "GET_SYS_PATH",
     "GET_ENV_PATH_ONELINER",
     "GET_PYTHON_VERSION_ONELINER",
-    "GET_PATHS_FOR_GENERIC_ENVS",
     "EnvError",
     "EnvCommandError",
     "IncorrectEnvError",

src/poetry/utils/env/base_env.py

@@ -175,13 +175,10 @@ def os(self) -> str:
     @property
     def site_packages(self) -> SitePackages:
         if self._site_packages is None:
-            # we disable write checks if no user site exist
-            fallbacks = [self.usersite] if self.usersite else []
             self._site_packages = SitePackages(
                 self.purelib,
                 self.platlib,
-                fallbacks,
-                skip_write_checks=not fallbacks,
+                self.fallbacks,
             )
         return self._site_packages
 
@@ -214,8 +211,14 @@ def platlib(self) -> Path:
 
         return self._platlib
 
+    @cached_property
+    def fallbacks(self) -> list[Path]:
+        paths = [Path(path) for path in self.paths.get("fallbacks", [])]
+        paths += [self.usersite] if self.usersite else []
+        return paths
+
     def _get_lib_dirs(self) -> list[Path]:
-        return [self.purelib, self.platlib]
+        return [self.purelib, self.platlib, *self.fallbacks]
 
     def is_path_relative_to_lib(self, path: Path) -> bool:
         for lib_path in self._get_lib_dirs():

src/poetry/utils/env/generic_env.py

@@ -8,7 +8,7 @@
 from typing import TYPE_CHECKING
 from typing import Any
 
-from poetry.utils.env.script_strings import GET_PATHS_FOR_GENERIC_ENVS
+from poetry.utils.env.script_strings import GET_PATHS
 from poetry.utils.env.virtual_env import VirtualEnv
 
 
@@ -78,7 +78,7 @@ def find_executables(self) -> None:
                 self._pip_executable = pip_executable
 
     def get_paths(self) -> dict[str, str]:
-        output = self.run_python_script(GET_PATHS_FOR_GENERIC_ENVS)
+        output = self.run_python_script(GET_PATHS)
 
         paths: dict[str, str] = json.loads(output)
         return paths

src/poetry/utils/env/script_strings.py

@@ -92,18 +92,16 @@ def _version_nodot(version):
 
 GET_PATHS = """\
 import json
-import sysconfig
-
-print(json.dumps(sysconfig.get_paths()))
-"""
-
-GET_PATHS_FOR_GENERIC_ENVS = """\
-import json
 import site
 import sysconfig
 
 paths = sysconfig.get_paths().copy()
 
+paths["fallbacks"] = [
+    p for p in site.getsitepackages()
+    if p and p not in {paths.get("purelib"), paths.get("platlib")}
+]
+
 if site.check_enableusersite():
     paths["usersite"] = site.getusersitepackages()
     paths["userbase"] = site.getuserbase()

src/poetry/utils/env/site_packages.py

@@ -25,7 +25,6 @@ def __init__(
         purelib: Path,
         platlib: Path | None = None,
         fallbacks: list[Path] | None = None,
-        skip_write_checks: bool = False,
     ) -> None:
         self._purelib = purelib
         self._platlib = platlib or purelib
@@ -40,7 +39,7 @@ def __init__(
             if path not in self._candidates:
                 self._candidates.append(path)
 
-        self._writable_candidates = None if not skip_write_checks else self._candidates
+        self._writable_candidates: list[Path] | None = None
 
     @property
     def path(self) -> Path:

tests/utils/env/test_env.py

@@ -309,7 +309,7 @@ def test_env_system_packages_are_relative_to_lib(
 
     # These are the virtual environments' base env packages,
     # in this case the system site packages.
-    for dist in metadata.distributions(path=[str(env.parent_env.site_packages.path)]):
+    for dist in env.parent_env.site_packages.distributions():
         assert (
             env.is_path_relative_to_lib(
                 Path(str(dist._path))  # type: ignore[attr-defined]