Why expected string become an tuple [duplicate]

Question

I expected the variable output_format to be a string. But when I ran the script it gave me a tuple type and threw an exception.

If I run in Python interpreter, it gave me an expected string.

('--sout "#standard{access=file,vcodec=h264,dst=c0_s0_h264_640x480_30_vbr_500_99_40000000.mp4}"',)
'h264'
<type 'str'>
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

Script source 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)

Answer 1

Your output_format is always tuple, because you put a comma after each possible value:

output_format = "..." % fname,
# ---------------------------^

Remove those commas and your cmd_str will once again only contain strings.

Python tuples are formed by such commas; the parenthesis are only needed when not using them would lead to ambiguity:

>>> 1
1
>>> 1,
(1,)
>>> type(_)
<class 'tuple'>