[Python-Dev] Usefulness of binary compatibility accross Python versions?

[Random832]

On Sat, Dec 16, 2017, at 08:22, Antoine Pitrou wrote:
> Typically, when adding a tp_XXX slot, you also need to add a
> Py_TPFLAGS_HAVE_XXX type flag to signal those static type structures
> that have been compiled against a recent enough PyTypeObject
> definition.  This way, extensions compiled against Python N-1 are
> supposed to "still work": as they don't have Py_TPFLAGS_HAVE_XXX set,
> the core Python runtime won't try to access the (non-existing) tp_XXX
> member.

Is there any practical for of having the flag off for one slot and on
for another slot that's been added later?

Could this be replaced (that is, a slot for such a thing added before
it's too late) with a simple counter that goes up with each version, and
any "unused" slot should have NULL or some other sentinel value? If it
really is important to have the flags themselves, just add another set
of flags - Py_TPFLAGS_HAVE_MORE_FLAGS.