Method __new__

example_code/item_062.py:156–161

(meta, name, bases, class_dict)

print("Example 6")
class ValidateFilled(type):
    def __new__(meta, name, bases, class_dict):
        # Only validate subclasses of the Filled class
        if bases:
            if class_dict["color"] not in ("red", "green"):
                raise ValueError("Fill color must be supported")
        return type.__new__(meta, name, bases, class_dict)

class Filled(metaclass=ValidateFilled):
    color = None  # Must be specified by subclasses