Descriptor protocol

Learn descriptors with __get__, __set__, __delete__.

What is the descriptor protocol?

A descriptor is an object that implements at least one of __get__, __set__ or __delete__. When used as a class-level attribute Python routes access through these methods.

For example if you write class Descriptor: def __get__(...) and Box.label = Descriptor(), reading box.label calls __get__. This mechanism underpins property, classmethod and super(). It is handy for validation, computation and caching.

The get signature

The signature is __get__(self, obj, objtype=None): obj is the instance being accessed and objtype is the class.

When accessed on the class (Box.label) obj is None; when accessed on an instance (box.label) obj is that instance. This distinction lets you behave differently at class level vs instance level. Most simple descriptors ignore objtype and only use obj.

Exercise Attribute access with a descriptor

Write a Descriptor class. Store the value with __init__(self, value). __get__(self, obj, objtype=None) returns self.value. In a Box class define label = Descriptor("kutu"). Create an instance and print the label attribute.

Show example solution

# Write the Descriptor class
class Descriptor:
    def __init__(self, value):
        self.value = value
    def __get__(self, obj, objtype=None):
        return self.value

class Box:
    label = Descriptor("kutu")

b = Box()
print(b.label)

Test yourself

Which method belongs to the descriptor protocol?
- __get__
- __init__
- __str__
- __repr__
Show answer

__get__ — __get__ is the read method of the descriptor protocol.
Match descriptor methods

__get__

Reading
__set__

Writing
__delete__

Deleting
descriptor

Class attribute

Complete the __get__ signature

def __get__(self, obj, ____):
    return self.value

Show answer

def __get__(self, obj, objtype=None):
    return self.value

Next lesson: Data descriptor

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