Read the "Releasing the buffer" section in PEP 3118 again. Unless I'm misunderstanding you completely, the rules you're asking for are already in place: those fields are entirely the responsibility of the exporting object, and it needs to ensure they remain valid until the buffer is released.

Now, it may be that we haven't *implemented* this correctly or consistently in the builtin objects, in which case we should do something about it. But the rules are there.