more path updates for unit tests

Author: timsaucer

python/tests/test_pickle_multiprocessing.py

@@ -40,17 +40,21 @@
 
 from . import _pickle_multiprocessing_helpers as helpers
 
-# `pytest --import-mode=importlib` (used in CI) does not prepend the test
-# parent directory to `sys.path`; pytest loads `tests` via its own importlib
-# hook. multiprocessing forkserver / spawn workers receive only the parent's
+# `pytest --import-mode=importlib` (used in CI) does not put the test parent
+# directory on `sys.path`; pytest loads `tests` via its own importlib hook.
+# multiprocessing forkserver / spawn workers receive only the parent's
 # `sys.path` snapshot, not pytest's hook, so they fail to import
 # `tests._pickle_multiprocessing_helpers` with `ModuleNotFoundError: No module
-# named 'tests'`. Ensure the parent directory of the `tests` package is on
-# `sys.path` so workers can resolve it the standard way. Fork start method is
-# unaffected (inherits the already-imported module object).
+# named 'tests'`. Append (not prepend) the parent directory of the `tests`
+# package so workers can resolve it the standard way, *without* shadowing the
+# installed `datafusion` wheel — the source tree's `python/datafusion/` has
+# no `_internal` extension module (that lives in the wheel under
+# site-packages), so prepending would break `from datafusion._internal
+# import ...`. Fork start method is unaffected (inherits the already-imported
+# module object).
 _TESTS_PARENT = str(Path(__file__).resolve().parent.parent)
 if _TESTS_PARENT not in sys.path:
-    sys.path.insert(0, _TESTS_PARENT)
+    sys.path.append(_TESTS_PARENT)
 
 
 @contextlib.contextmanager