title: Why expected string become an tuple
body: <p>I expected the variable <code>output_format</code> to be a string. But when I ran the script it gave me a <code>tuple</code> type and threw an exception.</p>
  
  <p>If I run in Python interpreter, it gave me an expected string.</p>
  
  <pre><code>('--sout "#standard{access=file,vcodec=h264,dst=c0_s0_h264_640x480_30_vbr_500_99_40000000.mp4}"',)
  'h264'
  &lt;type 'str'&gt;
  Traceback (most recent call last):
    File "streaming_verification/src/streaming_verification/scripts/streaming_verification.py", line 184, in run
      self.streaming.dump_file(export_fname, 5, codec_type)
    File "streaming_verification/src/streaming_verification/scripts/streaming.py", line 57, in dump_file
      cmd_str = " ".join(cmd)
  TypeError: sequence item 3: expected string, tuple found
  </code></pre>
  
  <p>Script source code:</p>
  
  <pre><code>def dump_file(self,
                fname='',
                period=10,
                codec_type="h264"):
  
      if "h264" == codec_type:
          output_format = "--sout \"#standard{access=file,vcodec=h264,dst=%s.mp4}\"" % fname,
      elif "mjpeg" == codec_type:
          output_format =  "--sout \"#standard{access=file,vcodec=mjpg ,dst=%s.avi}\"" % fname,
      elif "mpeg" == codec_type :
          output_format =  "--sout \"#standard{access=file,vcodec=h264,dst=%s.mp4}\"" % fname,
  
      pp(output_format)
  
      cmd =[
      "vlc",
      "-I dummy",
      "--rtsp-tcp {0}".format(self.conn_cfg["rtsp_link"]),
      output_format,
      "--stop-time {0} vlc://quit ".format(period)
      ]
      cmd_str = " ".join(cmd)
      self.run(cmd_str)
  </code></pre>