Why can't I replace the __str__ method of a Python object with another function?

Here is the code:

class Dummy(object):
     def __init__(self, v):
         self.ticker = v
 
 
 def main():
         def _assign_custom_str(x):
             def _show_ticker(t):                
                 return t.ticker
             x.__str__ = _show_ticker
             x.__repr__ = _show_ticker
             return x
 
 
     a = [Dummy(1), Dummy(2)]
 
     a1 = [_assign_custom_str(t) for t in a]
     print a1[1]
     # print a1[1].__str__ # test to if orig __str__ is replaced
 

I was hoping to see the output like this

2
 

However, instead I see the standard representation:

<__main__.Dummy object at 0x01237730>
 

Why?

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