Python 构造函数注入和PyMock

如何正确记录此示例（和简化）场景的模拟

class First:
    def register(self, handler):
        pass

class Second:
    def __init__(self, first):
        self.__first = first
        self.__first.register(self)

现在我有一个测试：

class SecondTest(PyMockTestCase):
    def test_registration(self):
        first = self.mock();
        first.register(?????) # what to pass as argument?
        self.replay()
        sut = Second(first)
        self.verify()