PEP 820 – PySlot: Unified slot system for the C API

[encukou (Petr Viktorin)]

@vstinner

If I understand correctly, all slots added after Python 3.15 should be used with PySlot_OPTIONAL. That’s not convenient. I would instead expect PyType_Slots() to ignore all “new” slots by default.

But if we add a new “mandatory” slot in Python 3.16, it should not be silently ignored by Python 3.15 (but fail with an error). In that case, I would prefer to mark such specific slot as PySlot_MANDATORY.

We could do that, but:

• The behaviour would be more error-prone: when you take a codebase and compile it for an earlier version of Python, some slots would be silently ignored. I’d much prefer only ignoring things if the user code explicitly says it’s OK.
• Currently, all slots are “mandatory”. To preserve the behaviour, when you convert from “old” slots to PySlot, all of them would need the flag.

@vstinner

I would prefer to remove PySlot_HAS_FALLBACK for now.

OK. This one can be added later, in the version that needs it.