🐛 fix(frontend): keep trailing spaces in backend paths

docs/changelog.d/+backend-path-whitespace.bugfix.rst

@@ -0,0 +1 @@
+Keep a trailing space in the last backend path when putting it on ``PYTHONPATH``.

src/pyproject_api/_via_fresh_subprocess.py

@@ -64,7 +64,7 @@ def __init__(
     @contextmanager
     def _send_msg(self, cmd: str, result_file: Path, msg: str) -> Generator[SubprocessCmdStatus, None, None]:  # ruff:ignore[unused-method-argument]
         env = os.environ.copy()
-        backend = os.pathsep.join(str(i) for i in self._backend_paths).strip()
+        backend = os.pathsep.join(str(i) for i in self._backend_paths)
         if backend:
             env["PYTHONPATH"] = backend
         process = Popen(  # ruff:ignore[subprocess-without-shell-equals-true]  # fixed argv, no shell

tests/test_frontend.py

@@ -55,6 +55,18 @@ def get_requires_for_build_wheel(config_settings=None):
     assert frontend.get_requires_for_build_wheel().requires == (Requirement("wheel"),)
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="Windows drops trailing spaces from names")
+def test_backend_path_with_trailing_space(tmp_path: Path) -> None:  # pragma: win32 no cover
+    backend: Final = tmp_path / "backend "
+    backend.mkdir()
+    (backend / "build_tester.py").write_text(
+        "def get_requires_for_build_wheel(config_settings=None):\n    return ['wheel']\n"
+    )
+    frontend: Final = SubprocessFrontend(tmp_path, (backend,), "build_tester", None, ())
+
+    assert frontend.get_requires_for_build_wheel().requires == (Requirement("wheel"),)
+
+
 @pytest.mark.parametrize(
     "hook",
     [