class Foo:
    el = 'el'

    def __init__(self):
        pass

class Bar(Foo):
    newel = []

    def __init__(self):
        pass

f = Foo()
b = Bar()
d = Bar()

b.newel.append(1)
b.newel.append(2)

print b.newel #[1, 2], OK
print d.newel #[1, 2], WTF?!
print f.newel #raises error: OK