From fc70bc09dfba784ef0baec24b325ef872ac354ac Mon Sep 17 00:00:00 2001
From: eavanvalkenburg <github@vanvalkenburg.eu>
Date: Mon, 2 Nov 2026 10:24:21 +0100
Subject: [PATCH] Python: Fix native changed-package path routing

---
 .github/workflows/python-tests.yml            |  6 ++
 .../scripts/run_tasks_in_changed_packages.py  | 13 ++--
 .../test_run_tasks_in_changed_packages.py     | 67 +++++++++++++++++++
 3 files changed, 80 insertions(+), 6 deletions(-)
 create mode 100644 python/scripts/tests/test_run_tasks_in_changed_packages.py

diff --git a/.github/workflows/python-tests.yml b/.github/workflows/python-tests.yml
index 5ae9ca2ce38..c2204ffddf8 100644
--- a/.github/workflows/python-tests.yml
+++ b/.github/workflows/python-tests.yml
@@ -99,6 +99,12 @@ jobs:
         run: uv run poe test -A --junitxml=pytest.xml
         working-directory: ./python
 
+      - name: Run changed-package routing tests
+        run: >-
+          uv run --locked --group dev --group tool-runtime
+          python -m pytest scripts/tests/test_run_tasks_in_changed_packages.py --import-mode=importlib -q
+        working-directory: ./python
+
       # Surface failing tests
       - name: Surface failing tests
         if: always()
diff --git a/python/scripts/run_tasks_in_changed_packages.py b/python/scripts/run_tasks_in_changed_packages.py
index 46e742c3a0c..93cd0c967f7 100644
--- a/python/scripts/run_tasks_in_changed_packages.py
+++ b/python/scripts/run_tasks_in_changed_packages.py
@@ -6,6 +6,7 @@
 from pathlib import Path
 
 from rich import print
+
 from scripts.task_runner import build_work_items, discover_projects, run_tasks
 
 # Tasks that need to run in all packages when core changes (type info propagates)
@@ -24,15 +25,15 @@ def get_changed_packages(
     core_package_changed = False
 
     for file_path in changed_files:
-        # Strip 'python/' prefix if present (when git diff is run from repo root)
-        file_path_str = str(file_path)
-        if file_path_str.startswith("python/"):
-            file_path_str = file_path_str[7:]  # Remove 'python/' prefix
+        # Strip the repository prefix using native separators so POSIX backslashes remain filename characters.
+        file_path_obj = Path(file_path)
+        if file_path_obj.parts and file_path_obj.parts[0] == "python":
+            file_path_obj = Path(*file_path_obj.parts[1:])
 
         # Convert to absolute path if relative
-        abs_path = Path(file_path_str)
+        abs_path = file_path_obj
         if not abs_path.is_absolute():
-            abs_path = workspace_root / file_path_str
+            abs_path = workspace_root / file_path_obj
 
         # Check which package this file belongs to
         for project in projects:
diff --git a/python/scripts/tests/test_run_tasks_in_changed_packages.py b/python/scripts/tests/test_run_tasks_in_changed_packages.py
new file mode 100644
index 00000000000..6a85156aa28
--- /dev/null
+++ b/python/scripts/tests/test_run_tasks_in_changed_packages.py
@@ -0,0 +1,67 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+"""Regression tests for changed-package path routing."""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import pytest
+
+from scripts.run_tasks_in_changed_packages import get_changed_packages
+
+PROJECTS = [Path("packages/core"), Path("packages/redis")]
+
+
+@pytest.mark.parametrize("package", ["core", "redis"])
+@pytest.mark.parametrize("spelling", ["forward", "native", "dot-forward", "dot-native"])
+def test_repository_relative_paths_select_package(tmp_path: Path, package: str, spelling: str) -> None:
+    workspace_root = tmp_path / "python"
+    relative = Path("python") / "packages" / package / "example.py"
+    paths = {
+        "forward": f"python/packages/{package}/example.py",
+        "native": str(relative),
+        "dot-forward": f"./python/packages/{package}/example.py",
+        "dot-native": f".{os.sep}{relative}",
+    }
+
+    assert get_changed_packages(PROJECTS, [paths[spelling]], workspace_root) == (
+        {Path("packages") / package},
+        package == "core",
+    )
+
+
+def test_existing_workspace_relative_and_absolute_paths_select_core(tmp_path: Path) -> None:
+    workspace_root = tmp_path / "python"
+    relative = Path("packages/core/example.py")
+    paths = [
+        relative.as_posix(),
+        str(relative),
+        f"./{relative.as_posix()}",
+        f".{os.sep}{relative}",
+        str(workspace_root / relative),
+    ]
+
+    for file_path in paths:
+        assert get_changed_packages(PROJECTS, [file_path], workspace_root) == ({PROJECTS[0]}, True), file_path
+
+
+@pytest.mark.skipif(os.name != "nt", reason="UNC paths use native Windows semantics.")
+def test_unc_absolute_path_selects_core() -> None:
+    workspace_root = Path(r"\\server\share\repo\python")
+    file_path = workspace_root / PROJECTS[0] / "example.py"
+
+    assert get_changed_packages(PROJECTS, [str(file_path)], workspace_root) == ({PROJECTS[0]}, True)
+
+
+@pytest.mark.skipif(os.name == "nt", reason="Backslash is a path separator on Windows.")
+def test_posix_literal_backslash_package_keeps_its_identity(tmp_path: Path) -> None:
+    workspace_root = tmp_path / "python"
+    literal_package = Path(r"packages/core\notes")
+    file_path = Path("python") / literal_package / "example.py"
+
+    assert get_changed_packages([*PROJECTS, literal_package], [str(file_path)], workspace_root) == (
+        {literal_package},
+        False,
+    )