Python Metaclasses Cheat Sheet

class Foo:    x = 1# Foo is itself an instance of typeprint(type(Foo))              # <class 'type'>print(isinstance(Foo, type))  # True# You can build an equivalent class dynamically with type()Foo2 = type("Foo2", (), {"x": 1})print(Foo2().x)   # 1# type(name, bases, namespace) is exactly what the `class` statement calls

class Meta(type):    def __new__(mcs, name, bases, namespace):        # Runs before the class object is created        for key, value in namespace.items():            if callable(value) and not key.startswith("__"):                print(f"defining method: {key}")        return super().__new__(mcs, name, bases, namespace)    def __init__(cls, name, bases, namespace):        # Runs after the class object is created        super().__init__(name, bases, namespace)class MyClass(metaclass=Meta):    def greet(self):        return "hi"# Prints "defining method: greet" at class-definition time

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 Config(metaclass=SingletonMeta):    def __init__(self):        self.settings = {}a = Config()b = Config()assert a is b   # Same instance every time