Metaprogramming

Metaprogramming is the technique of writing code that inspects, generates, or modifies other code at runtime. Python provides two primary mechanisms: decorators (function/class wrappers) and metaclasses (classes that create classes).

Decorators Recap

Decorators are the most common form of metaprogramming. They wrap a function or class to extend its behavior without modifying the original source code.

A decorator is simply a callable that takes a function and returns a new function. The @decorator syntax is sugar for func = decorator(func).

Metaclasses

A metaclass is the "class of a class." Just as an object is an instance of a class, a class is an instance of a metaclass. The default metaclass is type.

When you write class Foo: pass, Python internally calls type('Foo', (), {}) to create the class object. By supplying a custom metaclass, you intercept this creation step.

The Descriptor Protocol

Descriptors are objects that define __get__, __set__, and/or __delete__. They power Python's attribute access mechanism and are the foundation of @property, @classmethod, and @staticmethod.

Common Pitfalls

• Over-engineering: Don't use a metaclass when a decorator or __init_subclass__ suffices.
• Multiple metaclass conflicts: If class A has metaclass MA and class B has metaclass MB, a class inheriting both will raise TypeError.
• Debugging difficulty: Metaclass logic runs at class creation time, making stack traces confusing.