@Agent-Hellboy I don't understand how this test covers the problem described in the original issue... The problem in the issue is with handlers of loggers that are not supposed to be modified by new configuration, not with handlers defined in the configuration...

I would propose the following test:

def test_disable_existing_loggers(self):
        fn = make_temp_file(".log", "test_logging-disable-existing-loggers-")
        file_handler = logging.FileHandler(fn, mode="w")
        file_handler.setFormatter(logging.Formatter("%(message)s"))
        root_logger = logging.getLogger()
        root_logger.addHandler(file_handler)

        config = {"version": 1, "disable_existing_loggers": False}

        # we have disable_existing_loggers=False,
        # so, all handlers should continue working
        self.apply_config(config)

        msg = "test message"
        logging.warning(msg)
        file_handler.close()
        with open(fn, encoding='utf-8') as f:
            data = f.read().strip()
        os.remove(fn)
        self.assertEqual(data, msg)

okay, I was testing assert fileHandler in root_logger.handlers only, I have used yours test now

You should clarify that this is related to the logging module.

The reason why now the test is passing is that the root logger is not included in manager.loggerDict(). If you change the following line in the test

root_logger = logging.getLogger()

to

root_logger = logging.getLogger("any-logger")

the test will fail.

sorry, I simply copied your test without giving a thought

Shouldn't we call this _clearExistingHandlers() after we applied all the modifications from config, not before as it's happening now?

I have added a check

Should we rename the method to better reflect its behaviour? To something like _clearNotUsedHandlers.

the logging.shutdown() method calls handler.acquire(), handler.flush() and handler.release(). Also, it iterate all these handlers in a specific order (not sure why).

Don't we need all these actions here?

Also, it iterate all these handlers in a specific order (not sure why)

didn't get your question, it is to protect the handler in a concurrent environment. A mutex lock , acquire operation release

My question is why your implementation in this PR doesn't used them?