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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 start(self):
 """Start stream."""

        if not self.stream or self.stream.session.state == STATE_STOPPED:
            self.stream = RTSPClient(
                self.config.loop, self.stream_url, self.config.host,
                self.config.username, self.config.password,
                self.session_callback)
            self.stream.start()