problem descriptors in Python

[akbarza]

can you explain descriptors in Python?

A descriptor is an object that has a __get__() method and potentially also a __set__() and a __del__() method. This descriptor can be used as a member in another class to emulate a dynamic attribute.

In the below example, we create a MyDescriptor instance as the member 'spam' in the dictionary of class Thing. When we call vars(Thing)['spam'], we get the MyDescriptor instance that is stored in class Thing.

class MyDescriptor:
    def __get__(self, obj, objtype=None):
        return (self, obj, objtype)


class Thing:
    spam = MyDescriptor()


thing = Thing()

print(thing.spam)
print(Thing.spam)
print(vars(Thing)['spam'])

Output:
(<__main__.MyDescriptor object at 0x7f4e7f157fd0>, <__main__.Thing object at 0x7f4e7f157eb0>, <class '__main__.Thing'>)
(<__main__.MyDescriptor object at 0x7f4e7f157fd0>, None, <class '__main__.Thing'>)
<__main__.MyDescriptor object at 0x7f4e7f157fd0>

The magic of the __get__() method is that the expression Thing.spam evaluates to the return value of the descriptor's __get__() method called with 3 arguments: the descriptor itself, None and the Thing class.

When thing is an instance of Thing, the expression thing.spam evaluates to the return value of the descriptor's __get__() method called with 3 arguments: the descriptor itself, the Thing instance thing and the Thing class.

In the above example, the __get__() method returns its tuple of arguments, but it could return anything.

hi
can I ask you what is the __main__ that appears in all output in running above code?