Python Cookbook

Modifying the Class Hierarchy of an Instance

Credit: Ken Seehof

Problem

You need to modify the class hierarchy of an instance object that has already been instantiated.

Solution

A rather unusual application of the mix-in concept lets us perform this task in Python 2.0 or later (with some limitations in Python 2.2):

def adopt_class(klass, obj, *args, **kwds):
    're-class obj to inherit klass; call _ _init_ _ with *args, **kwds'
    # In Python 2.2, klass and obj._ _class_ _ must be compatible,
    # e.g., it's okay if they're both classic, as in the 'demo' function
    classname = '%s_%s' % (klass._ _name_ _, obj._ _class_ _._ _name_ _)
    obj._ _class_ _ = new.classobj(classname,  (klass, obj._ _class_ _), {})
    klass._ _init_ _(obj, *args, **kwds)

def demo(  ):
    class Sandwich:
        def _ _init_ _(self, ingredients):
            self.ingredients = ingredients
        def _ _repr_ _(self):
            return ' and '.join(self.ingredients)

    class WithSpam:
        def _ _init_ _(self, spam_count):
            self.spam_count = spam_count
        def _ _repr_ _(self):
            return Sandwich._ _repr_ _(self) + self.spam_count * ' and spam'

    pbs = Sandwich(['peanut butter', 'jelly'])
    adopt_class(WithSpam, pbs, 2)
    print pbs

Discussion

Sometimes class adoption, as illustrated by this recipe, is the cleanest way out of class hierarchy problems that arise when you wish to avoid module interdependencies (e.g., within a layered architecture). It’s more often useful if you want to add functionality to objects created by third-party modules, since modifying those modules’ source code is undesirable. ...