Metaclasses

How Classes Are Created

class X(Base) is not just a block of code — Python performs:

1. metaclass = most_derived_type(Base) (or type when no base defines one)
2. namespace = Meta.__prepare__("X", (Base,)) — the per-class namespace (a dict by default)
3. The class body executes into that namespace (__annotations__ included)
4. Meta.__new__(Meta, "X", (Base,), namespace) builds the class object
5. Meta.__init__(cls, name, bases, namespace) finishes setup

type itself is a metaclass — X = type("X", (Base,), {...}) is equivalent to the statement form. Every class is simultaneously an instance of its metaclass and (usually) an instance of type, which is why isinstance(X, Meta) and issubclass(X, Base) can both be true.

Writing a Metaclass

class Validated(type):
    def __new__(mcls, name, bases, ns, **kwargs):
        for field_name in ns.get("__annotations__", {}):
            if field_name.startswith("_"):
                raise TypeError(f"{name}.{field_name} cannot be annotated")
        cls = super().__new__(mcls, name, bases, ns, **kwargs)
        if hasattr(cls, "registry"):
            cls.registry[name] = cls
        return cls

    def __init__(cls, name, bases, ns, **kwargs):
        super().__init__(name, bases, ns, **kwargs)   # don't forget this

class Model(metaclass=Validated):
    registry: dict = {}

The same slot powers plugin registries: subclasses register the moment the class statement runs. ABCMeta (behind abc.ABC) enforces @abstractmethod at subclass creation — instantiating an incomplete class fails immediately, not at first call.

Pitfalls and Alternatives

Metaclass conflict. Two unrelated bases with different metaclasses:

TypeError: metaclass conflict: the metaclass of a derived class is the
most derived of its metaclasses

Fix by making one metaclass derive from the other, or restructure.

Prefer simpler tools:

Metaclasses pay off only for class-wide policy (registries, validation, ORMs like Django models). Cost: they compose badly with libraries that also want a metaclass, confuse readers, and interact subtly with __slots__. Interview angle: explain steps 1–5 of class creation, then justify why __init_subclass__ solved your last need without one.