flyweight.py

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

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

import weakref


class FlyweightMeta(type):
    def __new__(mcs, name, parents, dct):
        """

        :param name: class name
        :param parents: class parents
        :param dct: dict: includes class attributes, class methods,
        static methods, etc
        :return: new class
        """

        # set up instances pool
        dct['pool'] = weakref.WeakValueDictionary()
        return super(FlyweightMeta, mcs).__new__(mcs, name, parents, dct)

    @staticmethod
    def _serialize_params(cls, *args, **kwargs):
        """Serialize input parameters to a key.
        Simple implementation is just to serialize it as a string

        """
        args_list = map(str, args)
        args_list.extend([str(kwargs), cls.__name__])
        key = ''.join(args_list)
        return key

    def __call__(cls, *args, **kwargs):
        key = FlyweightMeta._serialize_params(cls, *args, **kwargs)
        pool = getattr(cls, 'pool', {})

        instance = pool.get(key)
        if not instance:
            instance = super(FlyweightMeta, cls).__call__(*args, **kwargs)
            pool[key] = instance
        return instance


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)


class Card2(object):
    __metaclass__ = FlyweightMeta

    def __init__(self, *args, **kwargs):
        # print('Init {}: {}'.format(self.__class__, (args, kwargs)))
        pass


if __name__ == '__main__':
    import sys
    if sys.version_info[0] > 2:
        sys.stderr.write("!!! This example is compatible only with Python 2 ATM !!!\n")
        raise SystemExit(0)

    # 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'))

    # Tests with metaclass
    instances_pool = getattr(Card2, 'pool')
    cm1 = Card2('10', 'h', a=1)
    cm2 = Card2('10', 'h', a=1)
    cm3 = Card2('10', 'h', a=2)

    assert (cm1 == cm2) != cm3
    assert (cm1 is cm2) is not cm3
    assert len(instances_pool) == 2

    del cm1
    assert len(instances_pool) == 2

    del cm2
    assert len(instances_pool) == 1

    del cm3
    assert len(instances_pool) == 0

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