O03, dunders and the slots behind them

[tamnd]

O02 opened the type struct and walked straight past the interesting part. In the middle of it are about seventy fields that are function pointers, and the interpreter reads them directly: repr(x) is a load out of Py_TYPE(x)->tp_repr and an indirect call.

But nobody writes tp_repr. You write __repr__. The thing in between is a table of ninety four rows in typeobject.c, and this lesson is about that table being walked in both directions.

What the lesson covers:

• backwards, add_operators turns every filled slot on a C type into a slot wrapper in the class dict, which is where int.__add__ and object.__repr__ come from, and type(int.__add__).__name__ is how you tell one from an ordinary method
• forwards, fixup_slot_dispatchers puts a generic dispatcher into every slot whose dunder you defined, and that dispatcher looks the name up on the type each time it runs
• which is the whole reason a __repr__ set on an instance is ignored: it really is in the instance dict, you can call it by name, and repr() goes through the slot and never looks there
• and the reason assigning one to a class works an hour later, on that class and every subclass, because type.__setattr__ calls update_slot
• __len__ fills mp_length and sq_length, which is why a class with only a length gets truthiness
• __getitem__ fills mp_subscript and sq_item, and sq_item is the old sequence protocol, so a class with one __getitem__ and nothing else is iterable and supports in
• __add__ and __radd__ share nb_add, so the dispatcher has the decision written into it: the reflected call goes first only if the right hand type is a subclass and defines the method itself rather than inheriting it
• and defining __eq__ without __hash__ is not a rule, it is overrides_hash stopping the slot being inherited and then type_ready_set_hash writing a literal None into the class dict, which you can look at

Everything is cited against v3.15.0rc1: slotdefs and the comment above it, TPSLOT, SLOT0, SLOT1BINFULL, add_operators, fixup_slot_dispatchers, update_slot, overrides_hash and type_ready_set_hash.

Six diagrams. Every cell prints exactly the same thing on 3.14, on 3.15 and in the browser probe, so nothing in this one needed a version note. The probe is at 31 lessons and 328 cells.

Two glossary terms went in: slot and slot wrapper. GLOSSARY.md is at 113.

Part of #23.