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

2019年8月3日 174次阅读

我在
python中实现了单例模式,但我注意到了

尽管返回了相同的实例,每次调用MyClass时都会调用init,这是无用的.

我怎么能避免呢？

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

最佳答案 问题是__new__不返回一个对象,它返回一个在其后调用__init__的单元化对象.

你根本无法避免这种情况.你可以做的是以下(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")
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