class Bidule(object):
   def __init__(self, name):
     self.name = name

def greet(self, who):
   print "hello %s, my name is %s" % (who, self.name)

b = Bidule('Bruno')
b.greet = greet
b.greet('Daniel')

=>
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "/tmp/python-23258Nq5.py", line 10, in <module>
     b.greet('Daniel')
TypeError: greet() takes exactly 2 arguments (1 given)

The point is that Python functions are descriptor objects that, when
looked up, return a (bound or unbound) method object wrapping
themselves. So to attach functions as method *on a per-instance basis*,
you either need to use new.instancemethod (as explained by Alex), or
directly use the descriptor protocol, ie:

b.greet = greet.__get__(b)
b.greet('Daniel')
=> hello Daniel, my name is Bruno

Note that you don't need this to attach a function as method to a class
- the descriptor protocol will then JustWork(tm).