descriptors.py

"""
Example showing that attribute lookups always require consulting the class,
even if an attribute is found in the instance __dict__. This is due to data
descriptors taking prescedence over instance attributes.
"""
class DataDescriptor:
    def __get__(self, inst, cls=None):
        return 'descriptor'
    def __set__(self, inst, value):
        pass

class MyClass:
    def __init__(self):
        self.foo = 'instance'
   
obj = MyClass()
assert obj.foo == 'instance'
MyClass.foo = DataDescriptor()
assert obj.foo == 'descriptor'
assert obj.__dict__['foo'] == 'instance'