flyweighting.py

import weakref

def generate_key(o):
    if hasattr(o, 'items'):
        return generate_key(sorted(o.items(), lambda a, b: cmp(a[0], b[0])))
    
    if hasattr(o, '__iter__'):
        return tuple(generate_key(v) for v in o)
    
    return o

class FlyweightedObject(object):
    _pool = weakref.WeakValueDictionary()

    def __new__(klass, *args, **kwargs):
        if not hasattr(klass.__init__, 'im_func'): raise 'cannot flyweight an object which has no python constructor'
            
        arguments = {}

        constructor = klass.__init__.im_func
        arguments_missing = constructor.func_code.co_argcount - len(args) - 1

        if arguments_missing > 0:
            args += constructor.func_defaults[-arguments_missing:]
        
        varnames = constructor.func_code.co_varnames[1:]
        
        for i in range(len(varnames)):
            varname = varnames[i]
            arguments[varname] = kwargs.get(varname, args[i])
        
        key = generate_key((klass, arguments))
        instance = klass._pool.get(key, None)

        if instance is None:
            instance = object.__new__(klass)
            klass._pool[key] = instance

        return instance

    def __getnewargs__(self):
        if hasattr(self.__class__.__init__, 'im_func'):
            constructor = self.__class__.__init__.im_func
            return tuple(getattr(self, attr) for attr in constructor.func_code.co_varnames[1:])
        return tuple()