Metaclasses & Introspection

A metaclass is the class of a class — the thing that creates class objects, just as a class creates instances. The default metaclass is type; you choose another with class Foo(metaclass=Meta).

A class is a first-class object: an instance of type, its metaclass. You can assign it to variables, pass it to functions, and build it at runtime. type(Foo) is type, and Foo.__class__ is its metaclass.

Introspection is a program examining an object's type and properties at runtime without changing it: type(obj), isinstance(obj, C), dir(obj), hasattr(obj, n), plus obj.__dict__ and obj.__class__. It reads, never mutates.

name is a runtime string, so getattr resolves names obj.name cannot express; if name is missing it returns default instead of raising AttributeError. It runs the same __getattribute__/__getattr__ lookup as obj.name. Siblings: setattr, delattr, hasattr.

When a class body finishes, Python calls the metaclass's __new__/__init__ with the name, bases, and namespace. It can inspect or modify the namespace, alter bases, register the class, then return the built class — once, at definition time.

Introspection only examines an object at runtime; reflection goes further and manipulates it — getting and setting attributes via getattr/setattr/delattr, calling methods, or creating objects by name, unknown at compile time.

Called with one argument, type(obj) returns the object's type. Called with three, type(name, bases, namespace) dynamically creates a new class — exactly what the class statement does under the hood via the default metaclass type.

Mostly to build declarative framework APIs: Django's models.Model metaclass turns class-level field declarations into DB descriptors and registers the model. Use them for registration, validation, or adding methods when a decorator is not enough.