how does Cpython implement its type Objects, i.e. type's type is always type?

Question

I understand that everything in python is an Object and that the \'type\' (or class) of these object is \'type\'. Plus the type of type is also type itself. (as explained nicely

Answer 1

The code that defines PyType_Type.ob_type = &PyType_Type involves a couple of indirections. It all starts in the function _Py_ReadyTypes() when it calls PyType_Ready(&PyType_Type). Before the function is called, the members tp_base and ob_type are both NULL. The function first sets type->tp_base to &PyBaseObject_Type (which is object in Python space) and then sets type->ob_type = PyBaseObject_Type.ob_type. The code uses Py_TYPE() which is just a macro for ob->ob_type. Since the type of object is type the code sets the type of type to type.

Now you have:

>>> type.__bases__
(<class 'object'>,)
>>> type(object)
<class 'type'>
>>> type(type(object))
<class 'type'>

The definition makes type an instance of object plus itself and object an instance of type.

>>> isinstance(type, object)
True
>>> isinstance(object, type)
True
>>> isinstance(type, type)
True

The type initialization code is much easier to understand in Python pseudo-code:

# object's class is type
object.__class__ = type
# PyType_Ready(type) sets:
type.__bases__ = (object,)
type.__class__ = type(object)