return an object of a different class

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How can I do something like this in python:

#!/usr/bin/python3.1

class MyNumbers:
   def __init__(self, n):
     self.original_value = n
     if n <= 100:
       self = SmallNumers(self)
     else:
       self = BigNumbers(self)

class SmallNumbers:
   def __init__(self, n):
     self.size = 'small'

class BigNumbers:
   def __init__(self, n):
     self.size = 'big'

t = MyNumbers(200)


When I do type(t) it says MyNumbers, while I'd want it to be BigNumbers, 
because BigNumbers and SmallNumbers will have different methods etc...

Do I need to use metaclasses?

Thanks.
-- 
Yves.                                                  http://www.SollerS.ca/
                                                        http://blog.zioup.org/