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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Unexpected SystemError in Python3.12.0a7: `\u003cmethod 'startswith' of 'str' objects\u003e returned a result with an exception set`","articleBody":"\u003c!--\r\n  If you're new to Python and you're not sure whether what you're experiencing is a bug, the CPython issue tracker is not\r\n  the right place to seek help. Consider the following options instead:\r\n\r\n  - reading the Python tutorial: https://docs.python.org/3/tutorial/\r\n  - posting in the \"Users\" category on discuss.python.org: https://discuss.python.org/c/users/7\r\n  - emailing the Python-list mailing list: https://mail.python.org/mailman/listinfo/python-list\r\n  - searching our issue tracker (https://github.com/python/cpython/issues) to see if\r\n    your problem has already been reported\r\n--\u003e\r\n\r\n# Bug report\r\n\r\nI noticed recently that [Setuptools CI started failing for Python 3.12.0a7](https://github.com/pypa/setuptools/actions/runs/4710601389/jobs/8354391872?pr=3893#step:7:1898) with a `SystemError` that is difficult to understand:\r\n\r\n\u003e `SystemError: \u003cbuilt-in method startswith of str object at 0x7fe399991298\u003e returned a result with an exception set`\r\n\r\nAt a first glance this error does not make much sense because `str.startswith` is a built-in method in a built-in data structure. I am struggling to see how it can have an exception set attached to the result.\r\n\r\nI managed to create the following simplified reproducer:\r\n\r\n```python\r\ndocker run --rm -it python:3.12.0a7-bullseye\r\nPython 3.12.0a7 (main, Apr 12 2023, 14:13:09) [GCC 10.2.1 20210110] on linux\r\nType \"help\", \"copyright\", \"credits\" or \"license\" for more information.\r\n\u003e\u003e\u003e class Distribution:\r\n...     @property\r\n...     def version(self):\r\n...         try:\r\n...             return self._version\r\n...         except AttributeError as e:\r\n...             # ... complex code omitted for the sake of simplification ...\r\n...             raise ValueError(\"Missing Version\")\r\n...     def __getattr__(self, attr):\r\n...         if attr.startswith(\"_\"):\r\n...             raise AttributeError(attr)\r\n...         # ... complex code omitted for the sake of simplification ...\r\n...         return 42\r\n...\r\n\u003e\u003e\u003e dist = Distribution()\r\n\u003e\u003e\u003e dist.version\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 5, in version\r\n  File \"\u003cstdin\u003e\", line 11, in __getattr__\r\nAttributeError: _version. Did you mean: 'version'?\r\n\r\nDuring handling of the above exception, another exception occurred:\r\n\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 8, in version\r\nValueError: Missing Version\r\n\r\nThe above exception was the direct cause of the following exception:\r\n\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\n  File \"\u003cstdin\u003e\", line 10, in __getattr__\r\nSystemError: \u003cmethod 'startswith' of 'str' objects\u003e returned a result with an exception set\r\n\u003e\u003e\u003e\r\n```\r\n\r\nThe error seems to be related to the combination of the following factors:\r\n1. A `property` is being used[^1]\r\n2. The property getter access an undefined attribute and triggers `__getattr__`\r\n3. `__getattr__` raises an `AttributeError`\r\n4. The property getter is supposed to catch the `AttributeError` exception and raise a different exeception.\r\n\r\nMy expectation is that  the `SystemError` never gets triggered with the confusing message. Instead the example should end up with a `ValueError`.\r\n\r\nPlease note that the example is very simplified. For the realistic implementation please consider [ `pkg_resources.Distribution`](https://github.com/pypa/setuptools/blob/v67.6.1/pkg_resources/__init__.py#L2628).\r\n\r\n# Your environment\r\n\r\n\u003c!-- Include as many relevant details as possible about the environment you experienced the bug in --\u003e\r\n\r\n- CPython versions tested on: 3.12.0a7\r\n- Operating system and architecture:\r\n  Reproducer tested on both Ubuntu 20.04.6 LTS machine and `python:3.12.0a7-bullseye` container\r\n\r\nProblem also identified in GitHub Actions runners: `ubuntu-latest`, `macos-latest`, `windows-latest` (see  https://github.com/pypa/setuptools/actions/runs/4710601389?pr=3893) for Python `3.12-dev`.\r\n\r\n\r\n[^1]: If we replace the property with a regular method, the example works as expected.\r\n","author":{"url":"https://github.com/abravalheri","@type":"Person","name":"abravalheri"},"datePublished":"2023-04-19T15:27:46.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/103632/cpython/issues/103632"}