Take a simple example in python:
>>> class A(object):
... pass
...
>>> def f(self):
... print "f called"
...
>>> A.f = f
>>> a = A()
>>> a.f()
f called
So in this example, the already existing class A
gets an additional (instance) function f
(though overriding existing ones works just as well). In real life this would of course happen e.g. in different modules. But how is this procedure called?