self.__foo = foo,雙下劃線為私有屬性

# 類的方法和屬性的訪問權限
class Test:
    def __init__(self, foo):
        self.__foo = foo

    def __bar(self):
        print(self.__foo)
        print('__bar')


# __bar,__foo無法在外部訪問
def main():
    test = Test('hello')
    test.__bar()  # AttributeError: 'Test' object has no attribute '__bar'
    print(test.__foo)   # AttributeError: 'Test' object has no attribute '__foo'


if __name__ == '__main__':
    main()