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Last active Mar 21, 2020
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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
return self.__default
# perform validation of value being set
if isinstance(value, str) and len(value.strip()) > 0:
self._instance_values[instance] = value
raise TypeError("The value provided is not a non-null string.")
if __name__ == "__main__":
student = Person()
studentB = Person()
studentC = Person()
print( # Default Name
print( # Default Name = "Padma"
print( # "Padma"
print( # Default Name = "Yu"
print( # Yu
print( # Default Name = "Vlad" # AttributeError: We've reached the maximum number of allowed instances.
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