data and non-data descriptors

Descriptors are special class attributes with a custom behavior on attribute get, set, or delete. If an object defines __set__ or __delete__, it is considered a data descriptor. Descriptors that only define __get__ are called non-data descriptors. The difference is that non-data descriptors are called only if the attribute isn't presented in __dict__ of the instance.

Non-data descriptor:

class D:
  def __get__(self, obj, owner):
    print('get', obj, owner)

class C:
    d = D()

c = C()
c.d
# get <C object at ...> <class 'C'>

# updating __dict__ shadows the descriptor
c.__dict__['d'] = 1
c.d
# 1

Data descriptor:

class D:
  def __get__(self, obj, owner):
    print('get', obj, owner)

  def __set__(self, obj, owner):
    print('set', obj, owner)

class C:
    d = D()

c = C()
c.d
# get <C object at ...> <class 'C'>

# updating __dict__ doesn't shadow the descriptor
c.__dict__['d'] = 1
c.d
# get <C object at ...> <class 'C'>