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Python單例模式的4種實現方法:
- #-*- encoding=utf-8 -*-
- print '----------------------方法1--------------------------'
- #方法1,實現__new__方法
- #並在將一個類的實例綁定到類變量_instance上,
- #如果cls._instance為None說明該類還沒有實例化過,實例化該類,並返回
- #如果cls._instance不為None,直接返回cls._instance
- class Singleton(object):
- def __new__(cls, *args, **kw):
- if not hasattr(cls, '_instance'):
- orig = super(Singleton, cls)
- cls._instance = orig.__new__(cls, *args, **kw)
- return cls._instance
-
- class MyClass(Singleton):
- a = 1
-
- one = MyClass()
- two = MyClass()
-
- two.a = 3
- print one.a
- #3
- #one和two完全相同,可以用id(), ==, is檢測
- print id(one)
- #29097904
- print id(two)
- #29097904
- print one == two
- #True
- print one is two
- #True
-
- print '----------------------方法2--------------------------'
- #方法2,共享屬性;所謂單例就是所有引用(實例、對象)擁有相同的狀態(屬性)和行為(方法)
- #同一個類的所有實例天然擁有相同的行為(方法),
- #只需要保證同一個類的所有實例具有相同的狀態(屬性)即可
- #所有實例共享屬性的最簡單最直接的方法就是__dict__屬性指向(引用)同一個字典(dict)
- #可參看:http://code.activestate.com/recipes/66531/
- class Borg(object):
- _state = {}
- def __new__(cls, *args, **kw):
- ob = super(Borg, cls).__new__(cls, *args, **kw)
- ob.__dict__ = cls._state
- return ob
-
- class MyClass2(Borg):
- a = 1
-
- one = MyClass2()
- two = MyClass2()
-
- #one和two是兩個不同的對象,id, ==, is對比結果可看出
- two.a = 3
- print one.a
- #3
- print id(one)
- #28873680
- print id(two)
- #28873712
- print one == two
- #False
- print one is two
- #False
- #但是one和two具有相同的(同一個__dict__屬性),見:
- print id(one.__dict__)
- #30104000
- print id(two.__dict__)
- #30104000
-
- print '----------------------方法3--------------------------'
- #方法3:本質上是方法1的升級(或者說高級)版
- #使用__metaclass__(元類)的高級python用法
- class Singleton2(type):
- def __init__(cls, name, bases, dict):
- super(Singleton2, cls).__init__(name, bases, dict)
- cls._instance = None
- def __call__(cls, *args, **kw):
- if cls._instance is None:
- cls._instance = super(Singleton2, cls).__call__(*args, **kw)
- return cls._instance
-
- class MyClass3(object):
- __metaclass__ = Singleton2
-
- one = MyClass3()
- two = MyClass3()
-
- two.a = 3
- print one.a
- #3
- print id(one)
- #31495472
- print id(two)
- #31495472
- print one == two
- #True
- print one is two
- #True
-
- print '----------------------方法4--------------------------'
- #方法4:也是方法1的升級(高級)版本,
- #使用裝飾器(decorator),
- #這是一種更pythonic,更elegant的方法,
- #單例類本身根本不知道自己是單例的,因為他本身(自己的代碼)並不是單例的
- def singleton(cls, *args, **kw):
- instances = {}
- def _singleton():
- if cls not in instances:
- instances[cls] = cls(*args, **kw)
- return instances[cls]
- return _singleton
-
- @singleton
- class MyClass4(object):
- a = 1
- def __init__(self, x=0):
- self.x = x
-
- one = MyClass4()
- two = MyClass4()
-
- two.a = 3
- print one.a
- #3
- print id(one)
- #29660784
- print id(two)
- #29660784
- print one == two
- #True
- print one is two
- #True
- one.x = 1
- print one.x
- #1
- print two.x
- #1
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