René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Exception chain lost when retry mechanism re-raises non-retryable errors","articleBody":"#### Environment details\n\n  - OS type and version: macOS 26.1\n  - Python version: 3.12.12\n  - pip version: 25.3\n  - `google-api-core` version: HEAD = [f8bf6f9610f3e0e7580f223794c3906513e1fa73](https://github.com/googleapis/python-api-core/commit/f8bf6f9610f3e0e7580f223794c3906513e1fa73)\n\n#### Description\n\nThe retry mechanism clears the `__cause__` attribute when re-raising non-retryable exceptions, breaking explicit exception chaining created with `raise ... from ...`.\n\n#### Steps to reproduce\n\n  1. Use the `@retry_async.AsyncRetry` decorator\n  2. Raise a non-retryable exception with a cause\n  3. Catch the exception and try to read or log its cause\n\n#### Code example\n\n```python\nimport asyncio\nfrom google.api_core import retry_async, exceptions\n\n@retry_async.AsyncRetry(predicate=retry_async.if_exception_type(exceptions.InternalServerError))\nasync def example():\n    try:\n        raise exceptions.Unauthenticated(\"Invalid credentials\")\n    except exceptions.Unauthenticated as exc:\n        raise ValueError(\"Access denied\") from exc  # Explicit chaining\n\nasync def main():\n    # After going through retry:\n    try:\n        await example()\n    except ValueError as e:\n        print(e.__cause__)  # Expected: \"401 Invalid credentials\"; Actual: None\n\nif __name__ == \"__main__\":\n    asyncio.run(main())\n```\n\nSee https://github.com/googleapis/python-api-core/pull/879/changes for a unit test that fails with the current code and passes with the proposed fix.\n\n#### Root Cause\n\nIn [retry_base.py:168](https://github.com/googleapis/python-api-core/blob/f8bf6f9610f3e0e7580f223794c3906513e1fa73/google/api_core/retry/retry_base.py#L168), `_default_exception_factory()` returns `(exc_list[-1], None)`, which causes raise `final_exc` from `None` on line 214, explicitly clearing `__cause__`.\n\n#### Proposed fix\n\nPreserve the exception's __cause__ attribute:\n\n```python\n# Before\nreturn exc_list[-1], None\n\n# After\nfinal_exc = exc_list[-1]\ncause = getattr(final_exc, '__cause__', None)\nreturn final_exc, cause\n```\n\nThis maintains exception chains for better debugging and meets developer expectations for exception handling.\n\n#### Draft pull request\n\nhttps://github.com/googleapis/python-api-core/pull/879","author":{"url":"https://github.com/fercor-cisco","@type":"Person","name":"fercor-cisco"},"datePublished":"2025-12-20T01:30:16.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/880/python-api-core/issues/880"}