python - 为什么 python 中的 Singleton 多次调用 __init__ 以及如何避免它？

Question

我已经在 python 中实现了单例模式，但我注意到init 被无用地调用，每次我调用 MyClass，尽管返回相同的实例。

如何避免？

class Test(object):
    def __init__(self, *args, **kwargs):
        object.__init__(self, *args, **kwargs)

class Singleton(object):
  _instance = None

  def __new__(cls):
    if not isinstance(cls._instance, cls):
        cls._instance = object.__new__(cls)
    return cls._instance

class MyClass(Singleton):
    def __init__(self):
        print("should be printed only 1 time")
        self.x=Test()
        pass

a = MyClass() # prints: "should be printed only 1 time"
b = MyClass() # prints ( again ): "should be printed only 1 time"

print(a,b) # prints: 0x7ffca6ccbcf8 0x7ffca6ccbcf8
print(a.x,b.x) # prints: 0x7ffca6ccba90 0x7ffca6ccba90

Answer 1

问题是 __new__ 不返回对象，它返回一个未初始化的对象，之后调用 __init__。

你根本无法避免。您可以执行以下操作(使用元类型):

class Singleton(type):
    def __init__(self, name, bases, mmbs):
        super(Singleton, self).__init__(name, bases, mmbs)
        self._instance = super(Singleton, self).__call__()

    def __call__(self, *args, **kw):
        return self._instance

class Test(metaclass = Singleton):
    # __metaclass__ = Singleton # in python 2.7
    def __init__(self, *args, **kw):
        print("Only ever called once")