weakref.py

from weakref import WeakKeyDictionary

class NonNullStringDescriptor:
    CAPACITY = 2  # maximum of two instances can store their non-default attribute values here

    def __init__(self, value: str = "Default Name"):
        self._instance_values = WeakKeyDictionary()
        self.__default = value

    def __get__(self, instance, owner):
        """
        Instead of directly returning the value directory, we look up each instance's value using the instance itself
        as the key.
        """
        if len(self._instance_values) < NonNullStringDescriptor.CAPACITY:
            self._instance_values[instance] = self.__default
        return self._instance_values.get(instance, self.__default)

    def __set__(self, instance, value):
        if instance not in self._instance_values and len(self._instance_values) >= NonNullStringDescriptor.CAPACITY:
            # if we are trying to add a new instance to descriptor instance store
            raise AttributeError("We've reached the maximum number of allowed instances.")
        elif instance in self._instance_values and value == self.__default:
            # if we are setting the instance value back to the default value, remove it from the weak ref dict
            self._instance_values.pop(instance)
            return self.__default

        # perform validation of value being set
        if isinstance(value, str) and len(value.strip()) > 0:
            self._instance_values[instance] = value
        else:
            raise TypeError("The value provided is not a non-null string.")

if __name__ == "__main__":
    student = Person()
    studentB = Person()
    studentC = Person()
    print(student.name)  # Default Name
    print(studentB.name)  # Default Name
    studentB.name = "Padma"
    print(studentB.name)  # "Padma"
    print(student.name)  # Default Name
    student.name = "Yu"
    print(student.name)  # Yu
    print(studentC.name) # Default Name
    studentC.name = "Vlad" # AttributeError: We've reached the maximum number of allowed instances.