Python编写只允许实例化一个对象的类

>>> class T:

    __total = 0

    def __init__(self, value):

        if T.__total != 0:

            raise Exception('You can only create one instanse')

        self.value = value

        T.__total += 1

>>> t1 = T(3)

# 实例化第二个对象时出错

>>> t2 = T(5)

Traceback (most recent call last):

  File "<pyshell#8>", line 1, in <module>

    t2 = T(5)

  File "<pyshell#6>", line 5, in __init__

    raise Exception('You can only create one instanse')

Exception: You can only create one instanse

但是，由于Python对私有数据成员没有提供严格的访问控制保护机制，所以上面的代码无法避免下面的情况。

>>> T._T__total = 0

>>> t2 = T(5)

>>> T._T__total = 0

>>> t3 = T(8)

>>> t1

<__main__.T object at 0x0000018771177FD0>

>>> t2

<__main__.T object at 0x0000018771177F98>

>>> t3

<__main__.T object at 0x00000187711F8F98>