How does __init_subclass__ provide a simpler alternative to metaclasses for many customization needs?

__init_subclass__(cls, **kwargs) is a classmethod automatically called whenever a subclass is defined, letting you hook into subclass creation (validation, registration, attribute injection) without writing a full metaclass. It's simpler because you just define it in the base class, and Python 3.6+ handles the rest -- covering the majority of use cases that used to require a custom metaclass.

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

class MyPlugin(Plugin):
    pass

print(Plugin.registry)  # [<class 'MyPlugin'>]