I suspect Cython wouldn’t immediately use it (at least for classes… obviously if it goes into PEP 793 then we would for that). Just because it’s mostly targeted at future improvements rather than an immediate new feature. It looks usable though.
I had one thought while reading it. I’m not yet sure if it’s a useful thought. In Steve’s PEP 809 he tried to make interfaces extensible with “namespaces”. Could that be useful here? I could see something like GraalPy wanting some custom type slots (e.g. an “inherits from this Java class” slot[1]). Similarly, at one point PyPy had an extra tp_pypy_flags in their typeobject struct.
I don’t know anything about Java so this is a totally arbitrary example that might have no link to reality ↩︎