generic_objects.py

# An example of separating behavior from data in Python

import copy, new

### Class helpers                                                               
def create_class(__bases__=(object, ), **kw):
    return new.classobj('FakeClass', __bases__, kw)

def combine(*classes):
    """ Create class hierarchies programatically """
    return new.classobj('CombinedClass', classes, {})

def add_methods(obj, **methods):
    """ Add methods to a class """
    for k, v in methods.items():
        setattr(obj.__class__, k, v)

def steal_methods(cls, *method_names):
    return dict((m, getattr(cls, m).im_func) for m in method_names)

### Instance helpers                                                            
class Generic(object):
    """ A generic class with no functionality, instances have private copies    
    of __class__ so behavior can be added to them programatically               
    """
    def __init__(self, **kw):
        self.__dict__ = dict(kw)
        self.__class__ = new.classobj('GenericInstance', (Generic,), {})

def add_behavior(obj, *classes):
    """ Adds each of the given classes to the bases for this object's class     
    Therefore inheriting any methods those classes define                       
    """
    obj.__class__.__bases__ = tuple(obj.__class__.__bases__ + classes)


### Time for a little test

# let's create two behavior classes                                                      
Adder = create_class(sum=lambda self: self.x + self.y)
Multiplier = create_class(mul=lambda self: self.x * self.y)

a1 = Generic(x=2, y=3)
# let's add a new method directly to a1
add_methods(a1, sum=lambda self: self.x + self.y)

print 'a1.sum() = ', a1.sum()

a2 = Generic(x=3, y=5)
# let's add behaviors to a2
add_behavior(a2, Adder, Multiplier)

print ''
print 'a2.sum() = ', a2.sum()
print 'a2.mul() = ', a2.mul()

a3 = Generic(x=5, y=7)
# let's steal some methods from the combination of Adder and Multiplier
# and add them to a3
add_methods(a3, **steal_methods(combine(Adder, Multiplier),
                                'sum', 'mul'))

print ''
print 'a3.sum() = ', a3.sum()
print 'a3.mul() = ', a3.mul()

#### Expected output of running the program
# a1.sum() =  5
# 
# a2.sum() =  8
# a2.mul() =  15
# 
# a3.sum() =  12
# a3.mul() =  35