Python VM Descriptors: Implementation Guide

In the Python virtual machine, a descriptor is a special object that can be used to define and control access to properties during the attribute access process in a class.

To implement a descriptor, you need to define a class and implement some specific methods within that class that are part of the descriptor protocol. The following are the methods that need to be implemented in the descriptor protocol:

1. This method is called when accessing an attribute through an instance. ‘self’ refers to the descriptor object itself, ‘instance’ refers to the instance object accessing the attribute, and ‘owner’ refers to the class object owning the attribute. This method should return the value of the attribute.
2. __set__(self, instance, value): This method is called when assigning a value to an attribute. ‘self’ refers to the descriptor object itself, ‘instance’ refers to the instance object accessing the attribute, and ‘value’ is the new value to be set. This method should perform the assignment operation on the attribute.
3. When an attribute is deleted, this method is called. “self” is the descriptor object itself, and “instance” is the instance object accessing the attribute. This method should delete the attribute.

Here is a simple example of a descriptor:

class Descriptor:
    def __get__(self, instance, owner):
        print("Getting value")
        return instance._value

    def __set__(self, instance, value):
        print("Setting value")
        instance._value = value

    def __delete__(self, instance):
        print("Deleting value")
        del instance._value

class MyClass:
    value = Descriptor()

my_obj = MyClass()
my_obj.value = 10
print(my_obj.value)
del my_obj.value

In the example above, the Descriptor class implements the methods of the descriptor protocol. The value attribute in the MyClass class uses the Descriptor descriptor. When accessing, setting, or deleting the value attribute, the corresponding methods of the descriptor are called. The output is as follows:

Setting value
Getting value
10
Deleting value