Adding a Method to an Existing Object Instance

Question

I\'ve read that it is possible to add a method to an existing object (i.e., not in the class definition) in Python.

I understand that it\'s not always good to do so

Answer 1

Module new is deprecated since python 2.6 and removed in 3.0, use types

see http://docs.python.org/library/new.html

In the example below I've deliberately removed return value from patch_me() function. I think that giving return value may make one believe that patch returns a new object, which is not true - it modifies the incoming one. Probably this can facilitate a more disciplined use of monkeypatching.

import types

class A(object):#but seems to work for old style objects too
    pass

def patch_me(target):
    def method(target,x):
        print "x=",x
        print "called from", target
    target.method = types.MethodType(method,target)
    #add more if needed

a = A()
print a
#out: <__main__.A object at 0x2b73ac88bfd0>  
patch_me(a)    #patch instance
a.method(5)
#out: x= 5
#out: called from <__main__.A object at 0x2b73ac88bfd0>
patch_me(A)
A.method(6)        #can patch class too
#out: x= 6
#out: called from