为什么要导入一个Python类来实例化它

这里有一个从一些PyTest单元测试中截取的例子，这些测试永远不会退出.如果我在DataListener.__init__上插入一个断点，我会看到它被test_Control.py中的from controller import Controller语句调用，这对我来说有点奇怪;为什么从模块导入一个类会实例化它的一个实例？

Date_listener.py

class DataListener:

    def __init__(self, port, host='localhost'):
        self._stop_monitor = Event()
        self._monitor = Thread(target=self.check_last_message)
        self._monitor.start()

    def __del__(self):
        self.stop()

    def stop(self):
        self._stop_monitor.set()
        self._monitor.join()

    def check_last_message(self):
        while not self._stop_monitor.is_set():
            # Code to check for a heartbeat here
            pass

Control.py

from data_listener import DataListener

class Controller:
    def __init__(
        self,
        subscriber=DataListener(port=DEFAULT_SUBSCRIBER_PORT),
        publisher=JsonPublisher(DEFAULT_PUBLISHER_PORT)):

        pass

test_Control.py

from controller import Controller

def test_controller_work_loop():
    # ARRANGE
    controller = Controller(subscriber=None, publisher=None)

    # Some actual tests here
    # This test passes and finishes but the test suite never exits