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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Breaking backward compatibility between `ctypes` and metaclasses in Python 3.13.","articleBody":"# Documentation\r\n\r\n\r\n## Background\r\nI am one of the maintainers of [`comtypes`](https://github.com/enthought/comtypes/). `comtypes` is based on `ctypes` and uses metaclasses to implement [`IUnknown`](https://learn.microsoft.com/en-us/windows/win32/api/unknwn/nn-unknwn-iunknown).\r\n\r\nIt was reported to the `comtypes` community that [an error occurs when attempting to use conventional metaclasses with Python 3.13](https://github.com/enthought/comtypes/issues/618#issuecomment-2366905288).\r\n\r\nA similar error was encountered in [`pyglet`](https://github.com/pyglet/pyglet), which also uses metaclasses to implement COM interfaces, when running on Python 3.13.\r\n\r\nBy referring to https://github.com/pyglet/pyglet/issues/1196 and https://github.com/pyglet/pyglet/pull/1199, I made several modifications to the code through trial and error, and now `comtypes` works in both Python 3.13 and earlier versions without problems:\r\n- https://github.com/enthought/comtypes/compare/a3a8733..04b766a\r\n  - It is necessary to pass arguments directly to the metaclass instead of using `__new__` in places where the metaclass is instantiated (i.e., where the class is dynamically defined). This also works in versions prior to Python 3.13.\r\n  - After calling `type.__new__`, any remaining initialization would be handled in `__init__` instead of in `__new__`. Since this results in an error in versions prior to Python 3.13, a bridge using `sys.version_info` is necessary.\r\n\r\nI think these changes are likely related to https://github.com/python/cpython/issues/114314 and https://github.com/python/cpython/issues/117142 and were introduced by the PRs linked to those issues.\r\n\r\nSince this change to `ctypes` breaks compatibility, I think it should be mentioned in the [What’s New In Python 3.13](https://docs.python.org/3.13/whatsnew/3.13.html) and/or in the [`ctypes` documentation](https://docs.python.org/3.13/library/ctypes.html).\r\nThere are likely other projects besides `comtypes` and `pyglet` that rely on the combination of `ctypes` and metaclasses, and I want to prevent confusion for those maintainers when they try to support Python 3.13.\r\n\r\n(Additionally, I would like to ask with the `ctypes` maintainers to confirm whether the changes for the metaclasses in `comtypes` (and `pyglet`) are appropriate.)\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-124546\n* gh-124708\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/junkmd","@type":"Person","name":"junkmd"},"datePublished":"2024-09-25T15:14:59.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":19},"url":"https://github.com/124520/cpython/issues/124520"}