import pymysql
from multiprocessing.pool import ThreadPool


def process(item):
    host = str(item).strip()
    db = None
    try:
        db = pymysql.connect(host=host, user='root', passwd='root', port=3306, connect_timeout=10,
                             write_timeout=10)
        r = open("success.txt", 'a')
        r.write(host)
        r.write("\n")
        pass
    except Exception as msg:
        print(msg)
        pass
    finally:

        if db:
            db.close()
            pass



if __name__ == '__main__':
        t = ThreadPool(100)
        t.map_async(process,open('3306.txt').readlines())
        t.close()
        t.join()
        

I want to verify in batches whether the MySQL account passwords of some IPs are valid, but my code will have the problem of sub-thread blocking. In fact, the last IP has been verified, but the main thread has not ended. I don't know how to modify it. You can provide Some modifications and suggestions?