Metaclasses trong Python là gì? Khi nào dùng?

# type() tạo class dynamically
MyClass = type('MyClass', (BaseClass,), {'attr': 42, 'method': lambda self: ...})

# Custom 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):
    def __init__(self):
        self.connection = None

db1 = Database()
db2 = Database()
assert db1 is db2  # True — cùng instance

# __init_subclass__ — thay thế nhẹ nhàng hơn metaclass
class PluginBase:
    _plugins = {}

    def __init_subclass__(cls, plugin_name: str, **kwargs):
        super().__init_subclass__(**kwargs)
        PluginBase._plugins[plugin_name] = cls

class MyPlugin(PluginBase, plugin_name="my_plugin"):
    pass