On Thu, Jul 2, 2020, at 20:39, William Pickard wrote:

I do have a plan to solve that problem, it involves adding about 24 more bytes to PyTypeObject, but it would allow Python to track offset from base object, total object size and best alignment value.

I'm not sure I'm following how that would help, but it sounds like you're saying that pointers to extension-defined data structure would not be PyTypeObject pointers, and vice versa? At least not without interconversion using this offset, which adds more overhead.