'Better way 50. __set_name__으로 클래스 애트리뷰트를 표시하라' 정리

# Example 11
class Field:
    def __init__(self):
        self.name = None
        self.internal_name = None

    def __set_name__(self, owner, name):
        print('__set_name__', owner, name)
        # Called on class creation for each descriptor
        self.name = name
        self.internal_name = '_' + name

    def __get__(self, instance, instance_type):
        if instance is None:
            return self
        return getattr(instance, self.internal_name, '')

    def __set__(self, instance, value):
        setattr(instance, self.internal_name, value)


# Example 12
print('\n==Before FixedCustomer==')
class FixedCustomer:
    first_name = Field()  # 요 때, __set_name__ 이 불림. name 이 first_name
    last_name = Field()
    prefix = Field()
    suffix = Field()


print('\n==Before cust==')
cust = FixedCustomer()
print(f'Before: {cust.first_name!r} {cust.__dict__}')  # Before: '' {}
cust.first_name = 'Mersenne'
print(f'After:  {cust.first_name!r} {cust.__dict__}')  # After:  'Mersenne' {'_first_name': 'Mersenne'}

==Before FixedCustomer==
__set_name__ <class '__main__.FixedCustomer'> first_name
__set_name__ <class '__main__.FixedCustomer'> last_name
__set_name__ <class '__main__.FixedCustomer'> prefix
__set_name__ <class '__main__.FixedCustomer'> suffix

==Before cust==
Before: '' {}
After:  'Mersenne' {'_first_name': 'Mersenne'}

# Example 7
class Field:
    def __init__(self):
        # These will be assigned by the metaclass.
        self.name = None
        self.internal_name = None

    def __get__(self, instance, instance_type):
        if instance is None:
            return self
        return getattr(instance, self.internal_name, '')

    def __set__(self, instance, value):
        setattr(instance, self.internal_name, value)


# Example 5
class Meta(type):
    def __new__(meta, name, bases, class_dict):
        for key, value in class_dict.items():
            if isinstance(value, Field):
                value.name = key
                value.internal_name = '_' + key
        cls = type.__new__(meta, name, bases, class_dict)
        return cls


# Example 6
class DatabaseRow(metaclass=Meta):
    pass


# Example 8
class BetterCustomer(DatabaseRow):
    first_name = Field()
    last_name = Field()
    prefix = Field()
    suffix = Field()


# Example 9
cust = BetterCustomer()
print(f'Before: {cust.first_name!r} {cust.__dict__}')  # Before: '' {}
cust.first_name = 'Euler'
print(f'After:  {cust.first_name!r} {cust.__dict__}')  # After:  'Euler' {'_first_name': 'Euler'}