Python類的實現（單實例、初始化一次）

 注：先調用__new__方法，後通過__init__初始化。

class Char(object):
    __instance = None
    __flag = None

    def __new__(cls, name):
        if cls.__instance == None:
            cls.__instance = object.__new__(cls)
            return cls.__instance
        else:
            return cls.__instance

    def __init__(self, name):
        if Char.__flag == None:
            self.name = name
            Char.__flag = self.name


a = Char("A")
print(id(a))
b = Char("B")
print(id(b))
print(b.name)