teste.py

import time

serials = {"COM1": "idle", "COM2": "idle"}

class Teste(object):
    
    def getIdle(self):

        for port in serials:
            if serials[port] == 'idle':
                serials[port] = 'busy'
                return port
        else:
            time.sleep(5)
            self.getIdle()