python单例模式的几种写法

1. 利用__metaclass__

这种写法的好处是，一处声明，到处引用，只要是想弄成单例的，直接设置就可以了，废话不说上代码

# 注意，继承type
class Singleton(type):

	def __call__(cls, *args, **kwargs):
		if not hasattr(cls, '_instance'):
			cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
		return cls._instance


class Test(object):
	# __metaclass__
	__metaclass__ = Singleton

t1 = Test()
t2 = Test()
print t1 == t2

想深入了解， 可以搜索python type详解，以及python类初始化的流程

2. 每个类单独写

这种写法好处是，可以给自己的单例类加上特有的逻辑

class MyTest(object):
	def __new__(cls, *args, **kwargs):
		if not hasattr(cls, '_instance'):
			# 两种写法，实际上都是为了调用object.__new__
			cls._instance = object.__new__(cls, *args, **kwargs)
			# cls._instance = super(MyTest, cls).__new__(cls, *args, **kwargs)
		return cls._instance

a = MyTest()
b = MyTest()
print a == b