Python 3 Deep Dive Part 4 Oop High Quality -

A metaclass is to a class what a class is to an instance. The default metaclass is type.

def my_meta(name, bases, dct):
    dct['version'] = 1.0
    return type(name, bases, dct)
class MyClass(metaclass=my_meta):
pass
print(MyClass.version)  # 1.0

Practical use: Singleton metaclass:

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]
class Database(metaclass=SingletonMeta):
pass

When to use metaclasses (rarely!):

High-quality rule: If you’re unsure, don’t use metaclasses. They make code magical and hard to debug. Prefer class decorators.

class ValidatePlugin(Plugin): name = "validator" python 3 deep dive part 4 oop high quality

def process(self, data):
    if "id" not in data:
        raise ValueError("Missing ID")
    return data

This design is extensible, testable, and respects LSP, OCP (Open/Closed), and DIP (Dependency Inversion).

When you use multiple inheritance, Python uses the C3 Linearization algorithm to determine which method is called. You can inspect this via __mro__. A metaclass is to a class what a class is to an instance