建造者模式，实例的参数初始化由建造类方法完成。

class Instance(object):
    def __init__(self, builder):
        super().__init__()
        self.url = builder.url()
        self.proxy = builder.proxy()


class Builder(object):
    def __init__(self):
        pass

    def url(self):
        return "url"

    def proxy(self):
        return "proxy"


def main():
    builder = Builder()
    instance = Instance(builder)


if __name__ == '__main__':
    main()