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Merge pull request faif#291 from gyermolenko/refactor_flyweight_pattern
Update flyweight pattern
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‎patterns/structural/flyweight__py3.py

Lines changed: 44 additions & 92 deletions
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,10 @@
1919
2020
*References:
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http://codesnipers.com/?q=python-flyweights
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https://python-patterns.guide/gang-of-four/flyweight/
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*Examples in Python ecosystem:
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https://docs.python.org/3/library/sys.html#sys.intern
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*TL;DR80
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Minimizes memory usage by sharing data with other similar objects.
@@ -27,109 +31,57 @@
2731
import weakref
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2933

30-
class FlyweightMeta(type):
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def __new__(mcs, name, parents, dct):
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"""
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Set up object pool
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:param name: class name
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:param parents: class parents
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:param dct: dict: includes class attributes, class methods,
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static methods, etc
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:return: new class
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"""
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dct['pool'] = weakref.WeakValueDictionary()
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return super(FlyweightMeta, mcs).__new__(mcs, name, parents, dct)
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@staticmethod
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def _serialize_params(cls, *args, **kwargs):
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"""
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Serialize input parameters to a key.
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Simple implementation is just to serialize it as a string
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"""
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args_list = list(map(str, args))
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args_list.extend([str(kwargs), cls.__name__])
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key = ''.join(args_list)
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return key
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def __call__(cls, *args, **kwargs):
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key = FlyweightMeta._serialize_params(cls, *args, **kwargs)
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pool = getattr(cls, 'pool', {})
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instance = pool.get(key)
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if instance is None:
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instance = super(FlyweightMeta, cls).__call__(*args, **kwargs)
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pool[key] = instance
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return instance
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6634
class Card(object):
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"""The Flyweight"""
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68-
"""The object pool. Has builtin reference counting"""
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_CardPool = weakref.WeakValueDictionary()
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"""Flyweight implementation. If the object exists in the
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pool just return it (instead of creating a new one)"""
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# Could be a simple dict.
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# With WeakValueDictionary garbage collection can reclaim the object
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# when there are no other references to it.
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_pool = weakref.WeakValueDictionary()
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7542
def __new__(cls, value, suit):
76-
obj = Card._CardPool.get(value + suit)
77-
if not obj:
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obj = object.__new__(cls)
79-
Card._CardPool[value + suit] = obj
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# If the object exists in the pool - just return it
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obj = cls._pool.get(value + suit)
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# otherwise - create new one (and add it to the pool)
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if obj is None:
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obj = object.__new__(Card)
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cls._pool[value + suit] = obj
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# This row does the part we usually see in `__init__`
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obj.value, obj.suit = value, suit
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return obj
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# If you uncomment `__init__` and comment-out `__new__` -
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# Card becomes normal (non-flyweight).
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# def __init__(self, value, suit):
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# self.value, self.suit = value, suit
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8658
def __repr__(self):
8759
return "<Card: %s%s>" % (self.value, self.suit)
8860

8961

90-
class Card2(metaclass=FlyweightMeta):
91-
def __init__(self, *args, **kwargs):
92-
# print('Init {}: {}'.format(self.__class__, (args, kwargs)))
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pass
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if __name__ == '__main__':
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# comment __new__ and uncomment __init__ to see the difference
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c1 = Card('9', 'h')
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c2 = Card('9', 'h')
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print(c1, c2)
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print(c1 == c2, c1 is c2)
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print(id(c1), id(c2))
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c1.temp = None
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c3 = Card('9', 'h')
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print(hasattr(c3, 'temp'))
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c1 = c2 = c3 = None
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c3 = Card('9', 'h')
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print(hasattr(c3, 'temp'))
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# Tests with metaclass
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instances_pool = getattr(Card2, 'pool')
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cm1 = Card2('10', 'h', a=1)
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cm2 = Card2('10', 'h', a=1)
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cm3 = Card2('10', 'h', a=2)
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assert (cm1 == cm2) and (cm1 != cm3)
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assert (cm1 is cm2) and (cm1 is not cm3)
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assert len(instances_pool) == 2
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del cm1
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assert len(instances_pool) == 2
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del cm2
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assert len(instances_pool) == 1
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del cm3
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assert len(instances_pool) == 0
129-
130-
### OUTPUT ###
131-
# (<Card: 9h>, <Card: 9h>)
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# (True, True)
133-
# (31903856, 31903856)
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# True
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# False
62+
def main():
63+
"""
64+
>>> c1 = Card('9', 'h')
65+
>>> c2 = Card('9', 'h')
66+
>>> c1, c2
67+
(<Card: 9h>, <Card: 9h>)
68+
>>> c1 == c2
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True
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>>> c1 is c2
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True
72+
73+
>>> c1.new_attr = 'temp'
74+
>>> c3 = Card('9', 'h')
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>>> hasattr(c3, 'new_attr')
76+
True
77+
78+
>>> Card._pool.clear()
79+
>>> c4 = Card('9', 'h')
80+
>>> hasattr(c4, 'new_attr')
81+
False
82+
"""
83+
84+
85+
if __name__ == "__main__":
86+
import doctest
87+
doctest.testmod()

‎patterns/structural/flyweight_with_metaclass__py3.py

Lines changed: 63 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,63 @@
1+
import weakref
2+
3+
4+
class FlyweightMeta(type):
5+
def __new__(mcs, name, parents, dct):
6+
"""
7+
Set up object pool
8+
9+
:param name: class name
10+
:param parents: class parents
11+
:param dct: dict: includes class attributes, class methods,
12+
static methods, etc
13+
:return: new class
14+
"""
15+
dct['pool'] = weakref.WeakValueDictionary()
16+
return super(FlyweightMeta, mcs).__new__(mcs, name, parents, dct)
17+
18+
@staticmethod
19+
def _serialize_params(cls, *args, **kwargs):
20+
"""
21+
Serialize input parameters to a key.
22+
Simple implementation is just to serialize it as a string
23+
"""
24+
args_list = list(map(str, args))
25+
args_list.extend([str(kwargs), cls.__name__])
26+
key = ''.join(args_list)
27+
return key
28+
29+
def __call__(cls, *args, **kwargs):
30+
key = FlyweightMeta._serialize_params(cls, *args, **kwargs)
31+
pool = getattr(cls, 'pool', {})
32+
33+
instance = pool.get(key)
34+
if instance is None:
35+
instance = super(FlyweightMeta, cls).__call__(*args, **kwargs)
36+
pool[key] = instance
37+
return instance
38+
39+
40+
class Card2(metaclass=FlyweightMeta):
41+
def __init__(self, *args, **kwargs):
42+
# print('Init {}: {}'.format(self.__class__, (args, kwargs)))
43+
pass
44+
45+
46+
if __name__ == '__main__':
47+
instances_pool = getattr(Card2, 'pool')
48+
cm1 = Card2('10', 'h', a=1)
49+
cm2 = Card2('10', 'h', a=1)
50+
cm3 = Card2('10', 'h', a=2)
51+
52+
assert (cm1 == cm2) and (cm1 != cm3)
53+
assert (cm1 is cm2) and (cm1 is not cm3)
54+
assert len(instances_pool) == 2
55+
56+
del cm1
57+
assert len(instances_pool) == 2
58+
59+
del cm2
60+
assert len(instances_pool) == 1
61+
62+
del cm3
63+
assert len(instances_pool) == 0

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