Josh Haberman <jhaber...@gmail.com> added the comment:
I know this is quite an old bug that was closed almost 10 years ago. But I am wishing this had been accepted; it would have been quite useful for my case. I'm working on a new iteration of the protobuf extension for Python. At runtime we create types dynamically, one for each message defined in a .proto file, eg. from "message Foo" we dynamically construct a "class Foo". I need to support class variables like Foo.BAR_FIELD_NUMBER, but I don't want to put all these class variables into tp_dict because there are a lot of them and they are rarely used. So I want to implement __getattr__ for the class, which requires having a metaclass. This is where the proposed PyType_FromSpecEx() would have come in very handy. The existing protobuf extension gets around this by directly calling PyType_Type.tp_new() to create a type with a given metaclass: https://github.com/protocolbuffers/protobuf/blob/53365065d9b8549a5c7b7ef1e7e0fd22926dbd07/python/google/protobuf/pyext/message.cc#L278-L279 It's unclear to me if PyType_Type.tp_new() is intended to be a supported/public API. But in any case, it's not available in the limited API, and I am trying to restrict myself to the limited API. (I also can't use PyType_GetSlot(PyType_Type, Py_tp_new) because PyType_Type is not a heap type.) Put more succinctly, I do not see any way to use a metaclass from the limited C API. Possible solutions I see: 1. Add PyType_FromSpecEx() (or similar with a better name) to allow a metaclass to be specified. But I want to support back to at least Python 3.6, so even if this were merged today it wouldn't be viable for a while. 2. Use eval from C to create the class with a metaclass, eg. class Foo(metaclass=MessageMeta) 3. Manually set FooType->ob_type = &MetaType, as recommended here: https://stackoverflow.com/a/52957978/77070 . Since PyObject.ob_type is part of the limited API, I think this might be possible! ---------- nosy: +jhaberman _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue15870> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
