Object Proxying (Python recipe) by tomer filiba

ActiveState Code (http://code.activestate.com/recipes/496741/)

class Proxy(object):
    __slots__ = ["_obj", "__weakref__"]
    def __init__(self, obj):
        object.__setattr__(self, "_obj", obj)
    
    #
    # proxying (special cases)
    #
    def __getattribute__(self, name):
        return getattr(object.__getattribute__(self, "_obj"), name)
    def __delattr__(self, name):
        delattr(object.__getattribute__(self, "_obj"), name)
    def __setattr__(self, name, value):
        setattr(object.__getattribute__(self, "_obj"), name, value)
    
    def __nonzero__(self):
        return bool(object.__getattribute__(self, "_obj"))
    def __str__(self):
        return str(object.__getattribute__(self, "_obj"))
    def __repr__(self):
        return repr(object.__getattribute__(self, "_obj"))
    
    #
    # factories
    #
    _special_names = [
        '__abs__', '__add__', '__and__', '__call__', '__cmp__', '__coerce__', 
        '__contains__', '__delitem__', '__delslice__', '__div__', '__divmod__', 
        '__eq__', '__float__', '__floordiv__', '__ge__', '__getitem__', 
        '__getslice__', '__gt__', '__hash__', '__hex__', '__iadd__', '__iand__',
        '__idiv__', '__idivmod__', '__ifloordiv__', '__ilshift__', '__imod__', 
        '__imul__', '__int__', '__invert__', '__ior__', '__ipow__', '__irshift__', 
        '__isub__', '__iter__', '__itruediv__', '__ixor__', '__le__', '__len__', 
        '__long__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', 
        '__neg__', '__oct__', '__or__', '__pos__', '__pow__', '__radd__', 
        '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', 
        '__repr__', '__reversed__', '__rfloorfiv__', '__rlshift__', '__rmod__', 
        '__rmul__', '__ror__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', 
        '__rtruediv__', '__rxor__', '__setitem__', '__setslice__', '__sub__', 
        '__truediv__', '__xor__', 'next',
    ]
    
    @classmethod
    def _create_class_proxy(cls, theclass):
        """creates a proxy for the given class"""
        
        def make_method(name):
            def method(self, *args, **kw):
                return getattr(object.__getattribute__(self, "_obj"), name)(*args, **kw)
            return method
        
        namespace = {}
        for name in cls._special_names:
            if hasattr(theclass, name):
                namespace[name] = make_method(name)
        return type("%s(%s)" % (cls.__name__, theclass.__name__), (cls,), namespace)
    
    def __new__(cls, obj, *args, **kwargs):
        """
        creates an proxy instance referencing `obj`. (obj, *args, **kwargs) are
        passed to this class' __init__, so deriving classes can define an 
        __init__ method of their own.
        note: _class_proxy_cache is unique per deriving class (each deriving
        class must hold its own cache)
        """
        try:
            cache = cls.__dict__["_class_proxy_cache"]
        except KeyError:
            cls._class_proxy_cache = cache = {}
        try:
            theclass = cache[obj.__class__]
        except KeyError:
            cache[obj.__class__] = theclass = cls._create_class_proxy(obj.__class__)
        ins = object.__new__(theclass)
        theclass.__init__(ins, obj, *args, **kwargs)
        return ins


------ example ------
>>> p = Proxy(6)
>>> p
6
>>> type(p)
<class '__main__.Proxy(int)'>
>>> p + 2
8
>>> p2 = Proxy([1,2,3])
>>> p2
[1, 2, 3]
>>> dir(p2)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__ #...
'__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__ini #...
educe__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', ' #...
'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
>>> isinstance(p2, list)
True
>>> p2.append(8)
>>> p2.append(2)
>>> p2.append(5)
>>> p2
[1, 2, 3, 8, 2, 5]
>>> p2.sort()
>>> p2
[1, 2, 2, 3, 5, 8]
>>> p2[2]
2
>>> p2[-1]
8
>>> type(p2)
<class '__main__.Proxy(list)'>
>>> p2.__class__
<type 'list'>

----- exceptions -----
Proxying user-types should work perfectly well. But proxying builtin objects,
like ints, floats, lists, etc., has some limitation and inconsistencies,
imposed by the interpreter:

>>> p = Proxy(6)
>>> p + p
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for +: 'Proxy(int)' and 'Proxy(int)'


>>> Proxy([1,2,3]) + [4,5]
[1, 2, 3, 4, 5]
>>> Proxy([1,2,3]) + Proxy([4,5])
>>> p = Proxy([1,2,3])
>>> p.extend(Proxy([4,5]))
>>> p
[1, 2, 3, 4, 5]
>>> p + Proxy([6, 7])
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "Proxy.py", line 49, in method
    return getattr(object.__getattribute__(self, "_obj"), name)(*args, **kw)
TypeError: can only concatenate list (not "Proxy(list)") to list


Also note that the methods of a proxied type return "real objects", not 
proxies. So, 

>>> p = Proxy(3)
>>> type(p)
<class '__main__.Proxy(int)'>
>>> p + 1
4
>>> type(_)
<type 'int'>
>>> p += 1
>>> p
4
>>> type(p)
<type 'int'>

In this case, 'p' was reassigned a real integer, and the proxy was
garbage-collected. What you might want to do is

>>> p = Proxy(3)
>>> p = Proxy(p + 1)
>>> p
4
>>> type(p)
<class '__main__.Proxy(int)'>