fix(cli): locate pyodide vendor site-packages by discovery, not a guessed OS-conditional path

packages/cli/src/pywrangler/sync.py

@@ -146,6 +146,26 @@ def create_pyodide_venv() -> None:
     run_command(["uv", "venv", str(pyodide_venv_path), "--python", interp_name])
 
 
+def _find_pyodide_site_packages() -> Path:
+    """Locate the pyodide venv's site-packages directory.
+
+    Discovered directly rather than assumed from an os.name check, since this
+    venv always targets a foreign (non-host) interpreter --
+    cpython-*-emscripten-wasm32-musl -- and there's no single formula that's
+    safe to assume holds for its internal layout on every host OS.
+    """
+    pyodide_venv_path = get_pyodide_venv_path()
+    matches = sorted(
+        pyodide_venv_path.glob("**/site-packages"), key=lambda p: len(p.parts)
+    )
+    if not matches:
+        raise RuntimeError(
+            f"Could not find a site-packages directory under {pyodide_venv_path}. "
+            "Try deleting .venv-workers and running `pywrangler sync` again."
+        )
+    return matches[0]
+
+
 def _install_requirements_to_vendor(
     plan: InstallPlan, allow_build: bool = False
 ) -> str | None:
@@ -177,11 +197,20 @@ def _install_requirements_to_vendor(
 
     # Clear pyodide venv site-packages so stale packages from previous syncs
     # don't carry over into python_modules.
-    pyv = get_python_version()
-    site_packages_path = (
-        f"lib/python{pyv}/site-packages" if os.name != "nt" else "Lib/site-packages"
-    )
-    pyodide_site_packages = get_pyodide_venv_path() / site_packages_path
+    #
+    # This venv always targets cpython-*-emscripten-wasm32-musl (see
+    # get_uv_pyodide_interp_name()), never the host's own interpreter -- unlike
+    # venv_workers_path (create_workers_venv()), which is a real native venv
+    # and does follow the host OS's own site-packages convention. Whether this
+    # *foreign*-target venv's internal layout also follows the host OS's
+    # convention, or the target's, isn't something to assume either way; it's
+    # located directly instead of guessed from os.name, which pointed at the
+    # wrong directory for at least one platform in practice (reported: real
+    # packages installed correctly by `uv`, but never copied into
+    # python_modules, so the deployed Worker fell back to workerd's
+    # always-raising stub "workers" module and failed at deploy time with
+    # ModuleNotFoundError).
+    pyodide_site_packages = _find_pyodide_site_packages()
     if pyodide_site_packages.is_dir():
         shutil.rmtree(pyodide_site_packages)
         pyodide_site_packages.mkdir()

packages/cli/tests/test_cli.py

@@ -309,6 +309,39 @@ def test_sync_removes_stale_packages(test_dir):
     )
 
 
+def test_find_pyodide_site_packages_discovers_nested_layout(test_dir):
+    """`_find_pyodide_site_packages` must locate site-packages by discovery,
+    not by constructing a path from a guessed OS-conditional convention.
+
+    The pyodide venv always targets a foreign (non-host) interpreter, so its
+    on-disk layout isn't guaranteed to match either the POSIX or the Windows
+    native-venv convention on every host -- this was previously assumed via
+    an `os.name` check, which pointed at a nonexistent directory on at least
+    one real platform (freshly-installed packages were silently never copied
+    into python_modules, producing the reported deploy-time
+    ModuleNotFoundError). Using an unconventional depth here (rather than
+    exactly mirroring either real convention) proves the discovery is
+    layout-agnostic, not just accidentally matching one specific guess.
+    """
+    pyodide_venv_path = pywrangler_sync.get_pyodide_venv_path()
+    real_site_packages = pyodide_venv_path / "unexpected" / "nested" / "site-packages"
+    real_site_packages.mkdir(parents=True)
+    (real_site_packages / "marker.txt").write_text("installed package marker")
+
+    found = pywrangler_sync._find_pyodide_site_packages()
+
+    assert found == real_site_packages
+    assert (found / "marker.txt").exists()
+
+
+def test_find_pyodide_site_packages_raises_when_missing(test_dir):
+    """A clear error, not a silent no-op or a copytree crash on a path that
+    doesn't exist, when the pyodide venv has no site-packages directory at
+    all (e.g. venv creation never completed)."""
+    with pytest.raises(RuntimeError, match="Could not find a site-packages"):
+        pywrangler_sync._find_pyodide_site_packages()
+
+
 def test_sync_lockfile_lifecycle(test_dir):
     """Test that pylock.toml pins versions and --upgrade refreshes them."""
     create_test_wrangler_jsonc(test_dir, "src/worker.py")