Descriptor behaviour

>>> class descr:
...   def __get__(self, obj, cls):
...     if obj is None:
...       print("Retrieved from class")
...     else:
...       print("Retrieved from instance")
...   def __set__(self, obj, val):
...     print("Set on instance")
...   def __delete__(self, obj):
...     print("Deleted from instance")
...
>>> class Demo:
...   attr = descr()
...
>>> Demo().attr
Retrieved from instance
>>> Demo().attr = 1
Set on instance
>>> del Demo().attr
Deleted from instance
>>> Demo.attr
Retrieved from class
>>> Demo.attr = 1
>>> Demo.attr
1
>>> Demo.attr = descr()
>>> Demo.attr
Retrieved from class
>>> del Demo.attr
>>> Demo.attr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Demo' has no attribute 'attr'