Class Variable Inheritance

[Brian Jones]

I'm sure the solution may be obvious, but this problem is driving me 
mad.  The following is my code:

class a(object):
        mastervar = []
        def __init__(self):
                print 'called a'
class b(a):
        def __init__(self):
                print 'called b'
                self.mapvar()
        def mapvar(self):
                self.mastervar.append(['b'])
class c(b):
        def __init__(self):
                print 'called c'
                self.mapvar()
        def mapvar(self):
                super(c, self).mapvar()
                self.mastervar.append(['c'])
if __name__ == '__main__':
        a1 = a()
        b1 = b()
        c1 = c()
        d1 = c() # Call C again
        print a1.mastervar
        print b1.mastervar
        print c1.mastervar
        print d1.mastervar
What I don't understand is why mastervar gets modified by each _seperate 
instance_ of classes that happen to extend the base class 'a'. 
Shouldn't mastervar be contained within the scope of the inheriting 
classes?  Why is it being treated like a global variable and being 
modified by the other instances?

Thanks,
Brian "bojo" Jones
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