flyweight.py

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

"""
*What is this pattern about?
This pattern aims to minimise the number of objects that are needed by
a program at run-time. A Flyweight is an object shared by multiple
contexts, and is indistinguishable from an object that is not shared.

The state of a Flyweight should not be affected by it's context, this
is known as its intrinsic state. The decoupling of the objects state
from the object's context, allows the Flyweight to be shared.

*What does this example do?
The example below sets-up an 'object pool' which stores initialised
objects. When a 'Card' is created it first checks to see if it already
exists instead of creating a new one. This aims to reduce the number of
objects initialised by the program.

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

*TL;DR80
Minimizes memory usage by sharing data with other similar objects.
"""

import weakref


class FlyweightMeta(type):
    def __new__(mcs, name, parents, dct):
        """
        Set up object pool

        :param name: class name
        :param parents: class parents
        :param dct: dict: includes class attributes, class methods,
        static methods, etc
        :return: new class
        """
        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 = 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 instance is None:
            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)


def with_metaclass(meta, *bases):
    """ Provide python cross-version metaclass compatibility. """
    return meta("NewBase", bases, {})


class Card2(with_metaclass(FlyweightMeta)):
    def __init__(self, *args, **kwargs):
        # print('Init {}: {}'.format(self.__class__, (args, kwargs)))
        pass


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'))

    # 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