Python descriptors

Question

From the descriptor docs:

A descriptor can be called directly by its method name. For example, d.__get__(obj).

What would be an example of this with the following class?

class Descriptor:
    def __init__(self, color="red"):
        self.color = color

For example, what is d and what is obj? How would "d.__get__(obj)" be called with the above class/instance?

Answer 1

Working Example

To make your example a descriptor, it needs to have a __get__() method:

class Descriptor:
    def __init__(self, color="red"):
        self.color = color
    def __get__(self, obj, objtype=None):
        return obj.size + ' ' + self.color

Use that descriptor in another class:

class A:
     pair = Descriptor('green')
     def __init__(self, size):
          self.size = size

Invoke the descriptor like this:

>>> a = A('big')
>>> a.pair
'big green'

Hope this working example helps :-)

Key points

1) A class is a descriptor if defines any one of __get__(), __set__(), or __delete__().

2) Put it to work by making an instance of the descriptor and storing it as a class variable in another class.

3) Invoke the descriptor with normal attribute lookup using the dot operator.

That's really all there is to it :-)