为什么导入Python类会实例化它?复制

这里有一个从一些永远不会退出的PyTest单元测试中截取的示例。如果我坚持一个断点 DataListener.__init__ 我看到它被呼叫 from controller import Controller 中的声明 test_controller.py 这让我有点吃惊;为什么从模块导入类会实例化它的实例?

data_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

控制器.py

from data_listener import DataListener

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

        pass

test_controller.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