python 單例

#-*- 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,
#若是cls._instance爲None說明該類尚未實例化過,實例化該類,並返回
#若是cls._instance不爲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徹底相同,能夠用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;所謂單例就是全部引用(實例、對象)擁有相同的狀態(屬性)和行爲(方法)
#同一個類的全部實例自然擁有相同的行爲(方法),
#只須要保證同一個類的全部實例具備相同的狀態(屬性)便可
#全部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是兩個不一樣的對象,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
#可是one和two具備相同的（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,
#單例類自己根本不知道本身是單例的,由於他自己(本身的代碼)並非單例的
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