class User(object):
    def __init__(self):
        self._user = None

    def __get__(self, instance, object_type=None):
        print 'Hi'
        self._user = 42
        return self._user

class Request(object):
    user = None

request = Request()
print request.user
# None

request.__class__.user = User()
print request.user
# Hi
# 42

request.user = User()
print request.user
# <__main__.User object at 0x00000000052DBC18>