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Chapter 24 — Architecture Notes: CPython Class Construction

1. The Dual Nature of type

type is the most unique construct in Python. It is both a class and an object.

  • It is an instance of itself: type(type) == type
  • It inherits from object: issubclass(type, object) == True
  • object is an instance of type: type(object) == type

This creates the core foundation of Python's object model: everything inherits from object, but everything is built by type.

2. The 5-Step Class Creation Sequence

When CPython reads a class definition during Evaluation Time, it executes a highly specific sequence:

class MyModel(BaseModel, metaclass=CustomMeta):
    data = 5
    def do_work(self): pass
  1. Metaclass Discovery: Python looks for metaclass= in the class signature. If none exists, it checks the parent classes. If none exist, it defaults to type.
  2. Prepare Namespace: Python calls CustomMeta.__prepare__(name, bases). This returns an empty dictionary. (In advanced cases, it returns an OrderedDict to preserve attribute declaration order).
  3. Evaluate Body: Python executes the code inside the class block. data = 5 and the do_work function are loaded into the dictionary returned by Step 2.
  4. Metaclass __new__: Python calls CustomMeta.__new__(CustomMeta, "MyModel", (BaseModel,), dict_from_step_3). This physically allocates the memory for the new Class Object.
  5. __init_subclass__: After the class is fully built, Python scans its parent classes (BaseModel) and calls their __init_subclass__ methods, allowing the parent to react to the newly born child.

3. Class Decorators vs Metaclasses

A Class Decorator executes after Step 5.

@add_timestamp
class MyModel: pass

# Under the hood, this is literally just:
MyModel = add_timestamp(MyModel)

Because it happens after Step 5, you cannot intercept or prevent the creation of the class. If the class definition was invalid, the memory was already allocated and the time was already wasted. A Metaclass (Step 4) intercepts the process before the memory is allocated.