On Wed, 08 Dec 2004 15:55:09 -0900, Brian Jones wrote: > 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
Brian, This is the first time I've responded to a post in this newsgroup so bear with me. First of all I believe that the place you declare mastervar is important. As you have placed it above (or outside) any class methods/functions it becomes a class variable. This gives it class scope and ALL instances of that class (and sub-classes I think) will reference the SAME variable. If, instead, you declare it inside the __init__ function of the class a object it will be an instance variable and inheritable. Secondly, in order to inherit methods and functions from base classes it is important to remember to call the base class __init__ function in the sub-class __init__ function. If you do both these things, that is declare mastervar as 'self.mastervar = []' inside class a.__init__ and call a.__init__ from b.__init__ (and likewise for b in c) then you should get the expected result. I hope. All I can say at this point is that I and not what you would call a programmer, and I could be quite wrong in my explanation. However if this doesn't help then there are many folk here who are better qualified to give you the correct solution. Regards -- http://mail.python.org/mailman/listinfo/python-list
