<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def poll(self):
 """ Poll Check for a non-response string generated by LCDd and return any string read. LCDd generates strings for key presses, menu events & screen visibility changes. """

        if select.select([self.tn], [], [], 0) == ([self.tn], [], []):
            response = urllib.unquote(self.tn.read_until(b"\n").decode())
            if self.debug: print "Telnet Poll: %s" % (response[:-1])
            # TODO Keep track of which screen is displayed
            return response
        else:
            return None