Advanced Metaclasses

Metaclass basics, new, init_subclass, and metaclass patterns.

Overview

Master metaclass concepts.

Metaclass Basics

class Meta(type):
    def __new__(cls, name, bases, attrs):
        # Add prefix to all methods
        for key, value in attrs.items():
            if callable(value) and not key.startswith('_'):
                attrs[f'custom_{key}'] = value
        
        return super().__new__(cls, name, bases, attrs)

class MyClass(metaclass=Meta):
    def method(self):
        return "original"

obj = MyClass()
print(obj.custom_method())  # original

init_subclass

class Plugin:
    plugins = []
    
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        Plugin.plugins.append(cls)

class PluginA(Plugin):
    pass

class PluginB(Plugin):
    pass

print(Plugin.plugins)  # [<class 'PluginA'>, <class 'PluginB'>]

Validation Metaclass

class ValidatedMeta(type):
    def __new__(cls, name, bases, attrs):
        for key, value in attrs.items():
            if isinstance(value, str) and not key.startswith('_'):
                attrs[key] = property(
                    lambda self, v=value: getattr(self, f'_{v}', None),
                    lambda self, v, key=key: setattr(self, f'_{key}', v)
                )
        return super().__new__(cls, name, bases, attrs)

class User(metaclass=ValidatedMeta):
    name = "str"
    email = "str"

u = User()
u.name = "Alice"
print(u.name)  # Alice

Practice

Create a metaclass that automatically adds logging to all methods.