env: enable usersite for system environments

Resolves: #2996

poetry/masonry/builders/editable.py

@@ -94,13 +94,7 @@ class EditableBuilder(Builder):
                 os.remove(str(setup))
 
     def _add_pth(self):
-        pth = self._env.site_packages.joinpath(self._module.name).with_suffix(".pth")
-        self._debug(
-            "  - Adding <c2>{}</c2> to <b>{}</b> for {}".format(
-                pth.name, self._env.site_packages, self._poetry.file.parent
-            )
-        )
-
+        pth_file = Path(self._module.name).with_suffix(".pth")
         paths = set()
         for include in self._module.includes:
             if isinstance(include, PackageInclude) and (
@@ -108,11 +102,33 @@ class EditableBuilder(Builder):
             ):
                 paths.add(include.base.resolve().as_posix())
 
-        with pth.open("w", encoding="utf-8") as f:
-            for path in paths:
-                f.write(decode(path + os.linesep))
+        content = ""
+        for path in paths:
+            content += decode(path + os.linesep)
+
+        for site_package in [self._env.site_packages, self._env.usersite]:
+            if not site_package:
+                continue
 
-        return [pth]
+            try:
+                site_package.mkdir(parents=True, exist_ok=True)
+                path = site_package.joinpath(pth_file)
+                self._debug(
+                    "  - Adding <c2>{}</c2> to <b>{}</b> for {}".format(
+                        path.name, site_package, self._poetry.file.parent
+                    )
+                )
+                path.write_text(content, encoding="utf-8")
+                return [path]
+            except PermissionError:
+                self._debug("- <b>{}</b> is not writable trying next available site")
+
+        self._io.error_line(
+            "  - Failed to create <c2>{}</c2> for {}".format(
+                pth_file.name, self._poetry.file.parent
+            )
+        )
+        return []
 
     def _add_scripts(self):
         added = []

poetry/utils/env.py

@@ -812,11 +812,15 @@ class Env(object):
     @property
     def site_packages(self):  # type: () -> Path
         if self._site_packages is None:
-            self._site_packages = Path(self.purelib)
-
+            self._site_packages = self.purelib
         return self._site_packages
 
     @property
+    def usersite(self):  # type: () -> Optional[Path]
+        if "usersite" in self.paths:
+            return Path(self.paths["usersite"])
+
+    @property
     def purelib(self):  # type: () -> Path
         if self._purelib is None:
             self._purelib = Path(self.paths["purelib"])
@@ -1017,6 +1021,8 @@ class SystemEnv(Env):
         # on some distributions it does not return the proper paths
         # (those used by pip for instance). We go through distutils
         # to get the proper ones.
+        import site
+
         from distutils.command.install import SCHEME_KEYS  # noqa
         from distutils.core import Distribution
 
@@ -1033,6 +1039,9 @@ class SystemEnv(Env):
 
             paths[key] = getattr(obj, "install_{}".format(key))
 
+        if site.check_enableusersite() and hasattr(obj, "install_usersite"):
+            paths["usersite"] = getattr(obj, "install_usersite")
+
         return paths
 
     def get_supported_tags(self):  # type: () -> List[Tag]

tests/utils/test_env.py

@@ -830,6 +830,17 @@ def test_system_env_has_correct_paths():
     assert env.site_packages == Path(paths["purelib"])
 
 
+@pytest.mark.parametrize(
+    ("enabled",), [(True,), (False,)],
+)
+def test_system_env_usersite(mocker, enabled):
+    mocker.patch("site.check_enableusersite", return_value=enabled)
+    env = SystemEnv(Path(sys.prefix))
+    assert (enabled and env.usersite is not None) or (
+        not enabled and env.usersite is None
+    )
+
+
 def test_venv_has_correct_paths(tmp_venv):
     paths = tmp_venv.paths