What is the __init_subclass__ pattern?

Question

Accepted Answer

**__init_subclass__** runs when a **class is subclassed**. Used for registration and validation.

```python
class Plugin:
    _plugins = {}

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

@classmethod
    def get(cls, name):
        return cls._plugins[name]

class TextPlugin(Plugin, plugin_name="text"):
    def process(self, data):
        return str(data)

class JsonPlugin(Plugin, plugin_name="json"):
    def process(self, data):
        import json
        return json.dumps(data)

# Auto-registration
print(Plugin._plugins)
# {"text": TextPlugin, "json": JsonPlugin}

plugin = Plugin.get("json")()
print(plugin.process({"key": "value"}))
```

**Use cases:**
- Plugin systems.
- ORM model registration.
- Validation at subclass creation.
- Replaces metaclass registration.

Python: What is the __init_subclass__ pattern?