Descriptors And Direct Access: Python Reference

The python 3.3 documentation tells me that direct access to a property descriptor should be possible, although I'm skeptical of its syntax x.__get__(a). But the example that I cons

Solution 1:

By accessing the descriptor on self you invoked __get__ already. The value 42 is being returned.

For any attribute access, Python will look to the type of the object (so type(self) here) to see if there is a descriptor object there (an object with a .__get__() method, for example), and will then invoke that descriptor.

That's how methods work; a function object is found, which has a .__get__() method, which is invoked and returns a method object bound to self.

If you wanted to access the descriptor directly, you'd have to bypass this mechanism; access x in the __dict__ dictionary of Owner:

>>>Owner.__dict__['x']
<__main__.MyDescriptor object at 0x100e48e10>
>>>Owner.__dict__['x'].__get__(None, Owner)
hello
42

This behaviour is documented right above where you saw the x.__get__(a) direct call:

The default behavior for attribute access is to get, set, or delete the attribute from an object’s dictionary. For instance, a.x has a lookup chain starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing through the base classes of type(a) excluding metaclasses.

The Direct Call scenario in the documentation only applies when you have a direct reference to the descriptor object (not invoked); the Owner.__dict__['x'] expression is such a reference.

Your code on the other hand, is an example of the Instance Binding scenario:

Instance Binding If binding to an object instance, a.x is transformed into the call: type(a).__dict__['x'].__get__(a, type(a)).