What is the __init_subclass__ for validation?

Question

Accepted Answer

**__init_subclass__** validates **class attributes** at subclass creation.

```python
class ValidatedModel:
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        # Validate required attributes
        for attr in ["table_name", "fields"]:
            if not hasattr(cls, attr):
                raise TypeError(f"{cls.__name__} must define {attr}")

# Validate field types
        if hasattr(cls, "fields"):
            for field_name, field_type in cls.fields.items():
                if not isinstance(field_type, type):
                    raise TypeError(f"Field {field_name} must be a type")

class User(ValidatedModel):
    table_name = "users"
    fields = {
        "name": str,
        "email": str,
        "age": int,
    }

class InvalidModel(ValidatedModel):
    pass  # TypeError: InvalidModel must define table_name

# Auto-registration with validation
class Handler:
    _handlers = {}

def __init_subclass__(cls, action=None, **kwargs):
        super().__init_subclass__(**kwargs)
        if action is None:
            raise ValueError("Handler must specify action")
        Handler._handlers[action] = cls

class CreateUser(Handler, action="create_user"):
    def execute(self, data):
        return User.objects.create(**data)
```

**Benefits:**
- Fail-fast validation.
- Clear error messages.
- Reusable validation logic.

Python: What is the __init_subclass__ for validation?