Use `new_attrs` when creating __slots__

datameta.py

class DataMeta(type):
    def __new__(cls, name, bases, attrs):
        def check_type(k, v):
            cls._validate(attrs[k]['type'], v)

        def set_default(self, kwargs):
            for key_meta, value_meta in attrs.items():
                if key_meta.startswith('__') and key_meta.endswith('__'):
                    continue
                if not isinstance(value_meta, dict):
                    continue

                if key_meta not in kwargs and 'default' in value_meta:
                    setattr(self, key_meta, value_meta['default'])
                elif key_meta not in kwargs and 'default' not in value_meta:
                    raise ValueError('{} is required.'.format(key_meta))

        def setattr_(self, k, v):
            if k in attrs:
                check_type(k, v)
            object.__setattr__(self, k, v)

        def init(self, **kwargs):
            for k, v in kwargs.items():
                setattr_(self, k, v)
            set_default(self, kwargs)

        new_attrs = {name: value for name, value in attrs.items() if name.startswith('__') or callable(value)}
        new_attrs['__slots__'] = [name for name in attrs if name not in new_attrs]
        new_attrs['__setattr__'] = setattr_
        new_attrs['__init__'] = init
        return type(name, bases, new_attrs)

    @staticmethod
    def _validate(type_, value):
        if not isinstance(value, type_):
            raise ValueError



class OrderData(metaclass=DataMeta):
    number = {'type': str}
    price = {'type': int}
    commission = {'type': float, 'default': 1.1}

    def get_total_price(self):
        return self.price * self.commission

    
if __name__ == '__main__':
    o = OrderData(number='10', price=100)
    print(o.number)
    print(o.commission)
    print(o.price)
    print(o.get_total_price())
    o.foo = 10