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单例模式的Python实现 #Algorithm #Python
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#-*- encoding=utf-8 -*- | |
print '----------------------方法1--------------------------' | |
#方法1,实现__new__方法 | |
#并在将一个类的实例绑定到类变量_instance上, | |
#如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回 | |
#如果cls._instance不为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检测 | |
print id(one) | |
#29097904 | |
print id(two) | |
#29097904 | |
print one == two | |
#True | |
print one is two | |
#True | |
print '----------------------方法2--------------------------' | |
#方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法) | |
#同一个类的所有实例天然拥有相同的行为(方法), | |
#只需要保证同一个类的所有实例具有相同的状态(属性)即可 | |
#所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict) | |
#可参看: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是两个不同的对象,id, ==, is对比结果可看出 | |
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(同一个__dict__属性),见: | |
print id(one.__dict__) | |
#30104000 | |
print id(two.__dict__) | |
#30104000 | |
print '----------------------方法3--------------------------' | |
#方法3:本质上是方法1的升级(或者说高级)版 | |
#使用__metaclass__(元类)的高级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的升级(高级)版本, | |
#使用装饰器(decorator), | |
#这是一种更pythonic,更elegant的方法, | |
#单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的 | |
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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