O05: what a dot does

[tamnd]

The fifth lesson of the object model part. O04 built the list of classes a name gets looked up in. This one is about the function that reads that list, and about the three other places it looks before and after.

The point of the lesson is that four separate rules people memorise about attributes are all the same hundred and twenty line function, read in order.

What is in it:

• _PyObject_GenericGetAttrWithDict does four things: walk the type's MRO, use what it found if that is a data descriptor, otherwise use the instance dict, otherwise fall back on what the MRO found and raise if there was nothing
• the same name put on the type and in the instance dict three different ways, showing that a data descriptor wins, a non data descriptor loses, and the instance dict entry is left alone either way
• which is also why obj.method = something shadows a method for one object while obj.some_property = something calls a setter instead
• __getattribute__ is the slot and runs for everything including __dict__, and the lesson shows an object you can no longer inspect through ordinary syntax because of it
• __getattr__ is not a slot at all, has no default, and gets a different dispatcher installed which removes itself from the slot when there is nothing to dispatch to
• a class lookup runs _Py_type_getattro_stackref instead, checks the metaclass MRO first, and calls any descriptor with None where the instance would be, which is the difference between Holder.watched and Holder().watched
• the two AttributeError messages are different because they come from different functions, and the did you mean suggestion is not in the message at all: it is computed from the obj attribute when a traceback is formatted
• the type attribute cache, four thousand and ninety six entries keyed on a version tag, with no invalidation pass, because zeroing tp_version_tag on a class and its subclasses is enough to make every entry stop matching
• and the observable end of that: the same LOAD_ATTR turning into LOAD_ATTR_INSTANCE_VALUE, LOAD_ATTR_SLOT, LOAD_ATTR_METHOD_WITH_VALUES or LOAD_ATTR_MODULE, then failing its guard when the class changes, dropping back to the general form, and settling on LOAD_ATTR_PROPERTY

Fourteen citations against v3.15.0rc1, including _PyObject_GenericGetAttrWithDict, _Py_slot_tp_getattr_hook, _Py_type_getattro_stackref, _PyType_LookupStackRefAndVersion, type_modified_unlocked, MCACHE_CACHEABLE_NAME, MAX_VERSIONS_PER_CLASS and PyDescr_IsData.

Six diagrams. One cell differs between 3.14 and 3.15, the wording of the did you mean suggestion, and it carries a note. Everything else prints the same on both and in the Pyodide browser probe, specialised opcodes included.

One glossary term: data descriptor. GLOSSARY.md is now 116.

Part of #23.