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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _mainthread_poll_readable(self):
 """Searches for readable client sockets. These sockets are then put in a subthread to be handled by _handle_readable """

        events = self._recv_selector.select(self.block_time)
        for key, mask in events:
            if mask == selectors.EVENT_READ:
                self._recv_selector.unregister(key.fileobj)

                self._threads_limiter.start_thread(target=self._subthread_handle_readable,
                                                   args=(key.fileobj,))