Add rule and checks to validate no test cross-imports are performed

.cursor/rules/pr-review.mdc

@@ -58,7 +58,13 @@ When adding a new weblog, verify the name is unique across all languages. Search
 
 - Ref: [build.md](mdc:docs/execute/build.md)
 
-## 10. Manifest YAML Syntax
+## 10. No Cross-Test-File Imports
+
+A `test_*.py` file must never import from another `test_*.py` file (absolute like `from tests.xxx.test_yyy import ...` or relative like `from .test_yyy import ...`). If logic is shared between test files, it must be moved to a local `utils.py` file in the same folder instead. Flag any PR that adds such an import.
+
+- Ref: [repository-structure.mdc](mdc:.cursor/rules/repository-structure.mdc), enforced by `tests/test_the_test/test_conventions.py::test_no_cross_test_file_imports`
+
+## 11. Manifest YAML Syntax
 
 - `bug` and `flaky` markers must include a JIRA ticket (e.g., `bug (JIRA-123)`)
 - Values with special YAML characters (`>`, `<`, `:`, `#`) must be quoted

.cursor/rules/repository-structure.mdc

@@ -119,4 +119,9 @@ system-tests/
   def test_XYZ(self):
       ...
   ```
-- Never define a setup method without a matching test method.
+- Never define a setup method without a matching test method.
+
+## 6. No Cross-Test-File Imports
+- A test file (`test_*.py`) must never import anything from another test file, whether via an absolute (`from tests.xxx.test_yyy import ...`) or relative (`from .test_yyy import ...`) import.
+- If logic needs to be shared between test files, move it into a local `utils.py` file in the same folder and have both test files import from there.
+- This is enforced by `tests/test_the_test/test_conventions.py::test_no_cross_test_file_imports`.

tests/test_the_test/test_conventions.py

@@ -1,5 +1,7 @@
+import ast
 import os
-from utils import scenarios
+from pathlib import Path
+from utils import scenarios, logger
 
 
 @scenarios.test_the_test
@@ -26,5 +28,71 @@ def test_utils():
             raise ValueError(f"File {os.path.join(folder, file)} is not a test file or a utils file {folder}")
 
 
+def _is_test_file(path: Path) -> bool:
+    return path.suffix == ".py" and path.name.startswith("test_")
+
+
+def _resolve_import_target(current_file: Path, module: str | None, level: int) -> Path | None:
+    """Resolve an import (relative or absolute) to the .py file path it points at, if any."""
+    if not module:
+        # `from . import something` / `from .. import something`: imports a package, not a specific file
+        return None
+
+    if level > 0:
+        directory = current_file.parent
+        for _ in range(level - 1):
+            directory = directory.parent
+        return directory.joinpath(*module.split(".")).with_suffix(".py")
+
+    if not module.startswith("tests."):
+        # not an import of another test module, nothing to resolve
+        return None
+
+    return Path(*module.split(".")).with_suffix(".py")
+
+
+@scenarios.test_the_test
+def test_no_cross_test_file_imports():
+    """A test file must never import from another test file. If logic is shared, it must live in a local utils.py."""
+
+    has_error = False
+
+    for folder, _, files in os.walk("tests"):
+        if folder.startswith("tests/fuzzer"):
+            # do not check these folders, they are particular use cases
+            continue
+
+        for file in files:
+            if not file.startswith("test_") or not file.endswith(".py"):
+                continue
+
+            path = Path(folder, file)
+
+            tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
+
+            for node in ast.walk(tree):
+                if isinstance(node, ast.ImportFrom):
+                    targets = [_resolve_import_target(path, node.module, node.level)]
+                elif isinstance(node, ast.Import):
+                    targets = [_resolve_import_target(path, alias.name, 0) for alias in node.names]
+                else:
+                    continue
+
+                for target in targets:
+                    if target is None or target == path:
+                        continue
+
+                    if _is_test_file(target):
+                        logger.error(f"{path} imports from another test file: {target}")
+                        has_error = True
+
+    if has_error:
+        raise ValueError(
+            "Some test file imports from another test file. "
+            "Shared logic must be moved to a local utils.py file instead."
+        )
+
+
 if __name__ == "__main__":
     test_utils()
+    test_no_cross_test_file_imports()