Example of using slotting a POPO class overriding __setattr__ to do validations

playing_slots.py

""" Just playing around with slots (BASIC EXAMPLE Plain Old Python Object) """
import re

date_pattern = re.compile(r'(\d{4})[/.-](\d{2})[/.-](\d{2})$')


class Person(object):
    """ Person sample class with slots """
    __slots__ = ('name', 'dob')

    def __init__(self):
        print(f'Initializing instance of {self}')

    @property
    def __dict__(self):
        """ Dictionary representation for a slotted class (that has no dict at all) """
        # Above works just for POPOs
        return {s: getattr(self, s, None) for s in self.__slots__}

    def __repr__(self):
        """ String representation of a slotted class using hijacked dict """
        return f'{self.__class__.__name__}({self.__dict__})'

    def __setattr__(self, key, value):
        print(f'Setting attribute "{key}" of {self} instance with {value} value')
        if key == 'dob' and not date_pattern.match(value):
            raise ValueError(f'Invalid date format "{value}" for {key} attribute')
        else:
            # Above is the trick to override __setattr__ in a slotted class
            super(Person, self).__setattr__(key, value)


if __name__ == '__main__':
    print(f'Running...')
    p1 = Person()
    p2 = Person()
    try:
        p1.name = 'Michael'
        p1.dob = '1988/03/22'
        p2.name = 'Rachel'
        # Above will crash since it does not match the pattern yyyy/mm/dd
        p2.dob = 'this-is-not-a-valid-date-format'
    except Exception as ex:
        print(f'{ex} was raised')
    finally:
        print(f'Person one is {p1}, person two is {p2}')