O02, following the type pointer

[tamnd]

O01 read the sixteen bytes at the front of an object and stopped at the second word, which is a pointer to the type. This lesson follows it.

What is on the other end is the largest struct in the interpreter, and almost all of it is answers to questions about instances rather than about the type. Most of those answers are readable from Python with no C at all.

What the lesson covers:

• following the type pointer from any object reaches type in at most two steps, and type points at itself, because the chain has to end somewhere and a loop is cheaper than a special case every lookup would have to check for
• tp_basicsize and tp_itemsize are the two numbers that say how big one instance is, and multiplying them out matches __sizeof__ for tuples and bytes and misses badly for list, whose items are a separate allocation
• type.__basicsize__ is nine hundred odd bytes because PyType_Type sets its own basicsize to sizeof(PyHeapTypeObject), so an instance of type is a class and a class carries all the operator tables inline
• the flags word tells a static type from a heap type, and Py_TPFLAGS_IMMUTABLETYPE is the entire reason int.nope = 1 raises and Plain.nope = 1 does not
• tp_dictoffset has four meanings: zero, a positive offset, a negative offset counted from the end of a var sized object, and exactly -1, which is a sentinel and not an offset, asserted against in the code that resolves the ordinary ones
• what -1 means is that the pointer lives in front of the object, three pointers back, with the weakref list one further back and the two words of GC pre header in between, which is why sys.getsizeof reports thirty two bytes more than an instance claims for itself
• and a class statement is a function call: the body compiles to a function, __build_class__ runs it, and the metaclass turns the namespace into a type, so type("Greeter", (), {"hi": hi}) comes out with the same flags, basicsize and mro

Everything is cited against v3.15.0rc1: _typeobject, PyHeapTypeObject, PyType_Type, Py_TPFLAGS_HEAPTYPE, tp_dictoffset, tp_weaklistoffset, _PyObject_ComputedDictPointer, MANAGED_DICT_OFFSET, _PyType_PreHeaderSize, _PyType_AllocNoTrack and builtin___build_class__.

Six diagrams. The notebook runs in Colab and all nine of its cells run in the browser probe, which is now at 30 lessons and 319 cells.

Two cells differ between 3.14 and 3.15 and both say so: type.__basicsize__ moved from 936 to 944, and the implicit return at the end of a module went from LOAD_CONST to LOAD_COMMON_CONSTANT. Two more carry a note about the browser, where a pointer is four bytes and __weakrefoffset__ reads minus sixteen.

Three glossary terms went in: static type, heap type and metaclass. type object picked up a citation and two cross references while I was there. GLOSSARY.md is at 111.

Part of #23.