Is it possible to change an instance's method implementation without changing all other instances of the same class? [duplicate]

Question

I do not know python very much (never used it before :D), but I can't seem to find anything online. Maybe I just didn't google the right question, but here I go:

I want to change an instance's implementation of a specific method. When I googled for it, I found you could do it, but it changes the implementation for all other instances of the same class, for example:

def showyImp(self):     print self.y  class Foo:     def __init__(self):         self.x = "x = 25"         self.y = "y = 4"      def showx(self):         print self.x      def showy(self):          print "y = woohoo"  class Bar:     def __init__(self):         Foo.showy = showyImp         self.foo = Foo()      def show(self):         self.foo.showx()         self.foo.showy()  if __name__ == '__main__':     b = Bar()     b.show()     f = Foo()     f.showx()     f.showy() 

This does not work as expected, because the output is the following:

x = 25

y = 4

x = 25

y = 4

And I want it to be:

x = 25

y = 4

x = 25

y = woohoo

I tried to change Bar's init method with this:

def __init__(self):     self.foo = Foo()     self.foo.showy = showyImp 

But I get the following error message:

showyImp() takes exactly 1 argument (0 given)

So yeah... I tried using setattr(), but seems like it's the same as self.foo.showy = showyImp.

Any clue? :)

Answer 1

Since Python 2.6, you should use the types module's MethodType class:

from types import MethodType  class A(object):     def m(self):         print 'aaa'  a = A()  def new_m(self):     print 'bbb'  a.m = MethodType(new_m, a) 

As another answer pointed out, however, this will not work for 'magic' methods of new-style classes, such as __str__().