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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`PyErr_SetObject()` behavior is strange and not as documented.","articleBody":"Briefly:\r\n`PyErr_SetObject(exc_type, exc_val)` does not create a new exception iff `isinstance(exc_val, BaseException)`, but uses `exc_val` instead.\r\n\r\nCallers of `PyErr_SetObject()` need various workarounds to handle this.\r\n\r\nThe long version:\r\n\r\nInternally CPython handles exceptions as a triple `(type, value, traceback)`, but the language treats exceptions as a single value.\r\n\r\nThis a legacy of the olden days before proper exceptions.\r\nTo handle adding proper exceptions to Python, various error handling functions, specifically `_PyErr_SetObject` still treat exceptions as triples, with the convention that if the value is an exception, then the exception is already normalized.\r\n\r\nOne other oddity is that if `exc_val` is a tuple, it is treated as the `*` arguments to `exc_type` when calling it. So, if `isinstance(exc_val, BaseException)` the desired behavior can be achieved by wrapping `exc_val` in a one-tuple.\r\n\r\nAs a consequence, both `_PyErr_SetKeyError` and `_PyGen_SetStopIterationValue` are a lot more complex than they should be to workaround this behavior.\r\n\r\nWe could make `PyErr_SetObject` act as documented, but that is likely to break C extensions, given how old this behavior is, and that it is relied on throughout CPython.\r\n\r\nCode that does the following is common:\r\n```C\r\n    exc = new_foo_exception();\r\n    PyErr_SetObject(\u0026PyFooException_Type, exc);\r\n```\r\n\r\nWe could just document the current behavior, but the current behavior is strange.\r\nWhat I suggest is this:\r\n* Create a new API function, PyErr_SetException(exc)` that takes a single exception object.\r\n* Document `PyErr_SetObject()` accurately\r\n* Deprecate the old function\r\n\r\n\r\nThis is an old bug going back to the 2 series.\r\n\r\n\u003c!-- gh-linked-prs --\u003e\r\n### Linked PRs\r\n* gh-101607\r\n* gh-101962\r\n* gh-102057\r\n* gh-102157\r\n* gh-102702\n* gh-113369\n* gh-113606\n\u003c!-- /gh-linked-prs --\u003e\r\n\r\nAlso relevant:\r\n* gh-102502","author":{"url":"https://github.com/markshannon","@type":"Person","name":"markshannon"},"datePublished":"2023-02-05T11:27:50.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":34},"url":"https://github.com/101578/cpython/issues/101578"}