types.MethodType

http://stackoverflow.com/questions/972/adding-a-method-to-an-existing-object-instance

532down voteaccepted +50

In Python, there is a difference between functions and bound methods. >>> def foo(): ... print "foo" ... >>> class A: ... def bar( self ): ... print "bar" ... >>> a = A() >>> foo <function foo at 0x00A98D70> >>> a.bar <bound method A.bar of <__main__.A instance at 0x00A9BC88>> >>> Bound methods have been "bound" (how descriptive) to an instance, and that instance will be passed as the first argument whenever the method is called. Callables that are attributes of a class (as opposed to an instance) are still unbound, though, so you can modify the class definition whenever you want: >>> def fooFighters( self ): ... print "fooFighters" ... >>> A.fooFighters = fooFighters >>> a2 = A() >>> a2.fooFighters <bound method A.fooFighters of <__main__.A instance at 0x00A9BEB8>> >>> a2.fooFighters() fooFighters Previously defined instances are updated as well (as long as they haven't overridden the attribute themselves): >>> a.fooFighters() fooFighters The problem comes when you want to attach a method to a single instance: >>> def barFighters( self ): ... print "barFighters" ... >>> a.barFighters = barFighters >>> a.barFighters() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: barFighters() takes exactly 1 argument (0 given) The function is not automatically bound when it's attached directly to an instance: >>> a.barFighters <function barFighters at 0x00A98EF0> To bind it, we can use the MethodType function in the types module: >>> import types >>> a.barFighters = types.MethodType( barFighters, a ) >>> a.barFighters <bound method ?.barFighters of <__main__.A instance at 0x00A9BC88>> >>> a.barFighters() barFighters This time other instances of the class have not been affected: >>> a2.barFighters() Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: A instance has no attribute 'barFighters'