为什么打印的是5 5 5 5而不是5 6 7？（多个单例调用）

Question

我正在尝试创建一个asingleton类，每次调用该类时，该类都会向“a”添加1：

class Singleton(object):
    _instance = None
    a=0
    def __new__(self):
        if not self._instance:
            self._instance = super(Singleton,self).__new__(self)
        self.a+=1
        return self._instance

x=Singleton()
x.a=5
print (x.a)
y=Singleton()
print (y.a)
z=Singleton()
print (z.a)

Answer 1

class Singleton(object):
    _instance = None
    a=0
    def __new__(self):
        if not self._instance:
            self._instance = super(Singleton,self).__new__(self)
        self._instance.a+=1
        return self._instance