Question

Why does the __get__ method in a python descriptor accept the owner class as it’s third argument? Can you give an example of it’s use?

The first argument (self) is self evident, the second (instances) makes sense in the context of the typically shown descriptor pattern (ex to follow), but I’ve never really seen the third (owner) used. Can someone explain what the use case is for it?

Just by way of reference and facilitating answers this is the typical use of descriptors I’ve seen:

class Container(object):
    class ExampleDescriptor(object):
        def __get__(self, instance, owner):
            return instance._name 
        def __set__(self, instance, value):
            instance._name = value
    managed_attr = ExampleDescriptor()

Given that instance.__class__ is available all I can think of is that explicitly passing the class has something to do with directly accessing the descriptor from the class instead of an instances (ex Container.managed_attr). Even so I’m not clear on what one would do in __get__ in this situation.

Answer 1

owner is used when the attribute is accessed from the class instead of an instance of the class, in which case instance will be None.

In your example attempting something like print(Container.managed_attr) would fail because instance is None so instance._name would raise an AttributeError.

You could improve this behavior by checking to see if instance is None, and it may be useful for logging or raising a more helpful exception to know which class the descriptor belongs to, hence the owner attribute. For example:

        def __get__(self, instance, owner):
            if instance is None:
                # special handling for Customer.managed_attr
            else:
                return instance._name