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#!/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__':
# 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