PyVarObject_HEAD_INIT vs PyObject_HEAD_INIT in "Defining extensions"

Core Development C API
1

I just read through this great tutorial:
2. Defining Extension Types: Tutorial — Python 3.10.8 documentation

I noticed that the CustomType uses PyVarObject_HEAD_INIT(NULL, 0) instead of PyObject_HEAD_INIT(NULL, 0). According to the reference, PyVarObject_HEAD_INIT is only needed for objects that have some notion of length, but the tutorial never adds a notion of length to CustomType. Would it be better for the example to use PyObject_HEAD_INIT, or am I misunderstanding the distinction?

The final example in the tutorial, SubListType, also uses PyVarObject_HEAD_INIT(NULL, 0), but that makes more sense to me since it is subclassing list.

2

PyTypeObject does have variable size. That’s the type of the custom object, not the custom object itself. (The individual custom objects are created in Custom_new rather than defined statically, so they don’t use a *_HEAD_INIT at all.)