flyweight.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""http://codesnipers.com/?q=python-flyweights"""

import weakref


class Card(object):

    """The object pool. Has builtin reference counting"""
    _CardPool = weakref.WeakValueDictionary()

    """Flyweight implementation. If the object exists in the
    pool just return it (instead of creating a new one)"""
    def __new__(cls, value, suit):
        obj = Card._CardPool.get(value + suit)
        if not obj:
            obj = object.__new__(cls)
            Card._CardPool[value + suit] = obj
            obj.value, obj.suit = value, suit
        return obj

    # def __init__(self, value, suit):
    #     self.value, self.suit = value, suit

    def __repr__(self):
        return "<Card: %s%s>" % (self.value, self.suit)

if __name__ == '__main__':
    # comment __new__ and uncomment __init__ to see the difference
    c1 = Card('9', 'h')
    c2 = Card('9', 'h')
    print(c1, c2)
    print(c1 == c2, c1 is c2)
    print(id(c1), id(c2))

    c1.temp = None
    c3 = Card('9', 'h')
    print(hasattr(c3, 'temp'))
    c1 = c2 = c3 = None
    c3 = Card('9', 'h')
    print(hasattr(c3, 'temp'))

### OUTPUT ###
# (<Card: 9h>, <Card: 9h>)
# (True, True)
# (31903856, 31903856)
# True
# False