fix: explicit PYTHONPATH for isolated test subprocesses (#593) (#594)

* fix: explicit PYTHONPATH for isolated test subprocesses (#593)

* fix: respect existing PYTHONPATH in subprocess tests

Put existing PYTHONPATH first, then append repo_root as fallback.
This is less invasive to user environments while still ensuring
the local package is available when needed.

Author: planetf1

docs/examples/conftest.py

@@ -498,12 +498,25 @@ def __init__(self, **kwargs):
         super().__init__(**kwargs)
 
     def runtest(self):
+        import os
+        import pathlib
+
+        repo_root = str(pathlib.Path(__file__).parent.parent.parent.resolve())
+        env = os.environ.copy()
+        existing_pythonpath = env.get("PYTHONPATH", "")
+        env["PYTHONPATH"] = (
+            f"{existing_pythonpath}{os.pathsep}{repo_root}"
+            if existing_pythonpath
+            else repo_root
+        )
+
         process = subprocess.Popen(
             [sys.executable, self.path],
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             text=True,
             bufsize=1,  # Enable line-buffering
+            env=env,
         )
 
         # Capture stdout output and output it so it behaves like a regular test with -s.

test/conftest.py

@@ -251,6 +251,11 @@ def _run_heavy_modules_isolated(session, heavy_modules: list[str]) -> int:
         if markexpr:
             cmd.extend(["-m", markexpr])
 
+        import pathlib
+
+        repo_root = str(pathlib.Path(__file__).parent.parent.resolve())
+        env["PYTHONPATH"] = f"{repo_root}{os.pathsep}{env.get('PYTHONPATH', '')}"
+
         # Stream output in real-time while capturing for parsing
         process = subprocess.Popen(
             cmd,