What is the __init_subclass__ method?

Question

Accepted Answer

**__init_subclass__** is called when a **class is subclassed**. Useful for registration and validation.

```python
class Plugin:
    _registry = {}

def __init_subclass__(cls, plugin_name=None, **kwargs):
        super().__init_subclass__(**kwargs)
        name = plugin_name or cls.__name__.lower()
        Plugin._registry[name] = cls

@classmethod
    def get_plugin(cls, name):
        return cls._registry.get(name)

class AudioPlugin(Plugin, plugin_name="audio"):
    def process(self):
        return "Processing audio"

class VideoPlugin(Plugin, plugin_name="video"):
    def process(self):
        return "Processing video"

print(Plugin._registry)
# {"audio": AudioPlugin, "video": VideoPlugin}

plugin = Plugin.get_plugin("audio")()
print(plugin.process())  # "Processing audio"
```

**Use cases:**
- Auto-registration.
- Validation at subclass creation.
- Plugin systems.
- Replaces metaclass registration pattern.

Python: What is the __init_subclass__ method?