How to use descriptors and slots in Python at the same time

Question

I am new to your community. Sorry for my English. I am interested in Python languages. Please help me understand descriptors. I don't know how to use descriptors and _slots_ in class Calendar at the same time. Here is an example of my code:

    class dateValue:
    def __checkValue(value):
        if isinstance(value, int) or isinstance(value, float):
            return True
        return False

    def __set_name__(self, owner, name):
        self.__name = name

    def __get__(self, instance, owner):
        return instance.__dict__[self.__name]

    def __set__(self, instance, value):
        if dateValue.__checkValue(value):
            instance.__dict__[self.__name] = value
        else:
            raise ValueError("Неверный формат данных")

    def __delete__(self, instance):
        del instance.__dict__[self.__name]

    class Calendar:
    
        #__slots__ = ["day", "month", "year"]
    
        day = dateValue()
        month = dateValue()
        year = dateValue()
    
        def __init__(self, day=0, month=0, year=0):
            self.day = day
            self.month = month
            self.year = year

cal4 = Calendar()
cal4.day = 3
cal4.month = 3
cal4.year = 2023

day4 = cal4.day
month4 = cal4.month
year4 = cal4.year

print(day4, month4, year4, sep=".")

*stop using double-underscore name mangling*. Why are you doing that? — juanpa.arrivillaga, Jan 02 '21 at 08:24
Anyway, the whole point of `__slots__` is that instances don't have a `__dict__`. Addtionally, `__slots__` *work through* descriptors, so you can't use class variables with the same name. If you just create an empty class but with those slots, and look at `vars(MyClass)` you'll see that they are there — juanpa.arrivillaga, Jan 02 '21 at 08:39

score 1 · Answer 1 · edited Oct 27 '21 at 01:34

BLUF: if Calendar uses __slots__, it won't have a __dict__ attribute (that's the whole point of slots).

Per the python language reference (Notes on using __slots__):

__slots__ are implemented at the class level by creating descriptors (Implementing Descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by __slots__; otherwise, the class attribute would overwrite the descriptor assignment.

There are two things you can do to fix the error and keep your code working (mainly) the same way.

Don't use __slots__. They deny the creation of a __dict__, and you want the __dict__.
(What I prefer), your class dateValue can own the underlying value and return it on __get__, instead of replacing the accessor with the value. your Calendar can keep its __slots__ and offload the additional values out of sight.

How to use descriptors and __slots__ in Python at the same time

1 Answers1

How to use descriptors and slots in Python at the same time