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#!/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