class KlassA(object):

    def __set__(self, instance, value):
        print 'Setting value'
        self.value = value

class KlassB(object):

    a = KlassA()

    def __init__(self):
        print 'Working'

class KlassC(object):

    def __init__(self):
        self.a = KlassA()
        print 'Not working'

b = KlassB()
b.a = 'hej'
print type(b.a)
print b.a.value

c = KlassC()
c.a = 'hejsan'
print type(c.a)
print c.a.value