Difference between self and a reference to own class. (unique instance)

[Dupletor]

class external(threading.Thread):
    i = 0
    def __init__(self, dir_q, result_q):
        super(external, self).__init__()

then right after "super", I write:

    self.i = 5

--external.i returns 0 (from another object)

    external.i = 5

--external.i returns 5 (from another object)

What is the difference, if only one instance of external exists?

By the way, self.i returns 5 if executed inside of a method from this instance in either way. If the instances sees self.i equal to external.i, why do other classes see external.i different that what this sees as self.i?