为什么期望字符串成为一个元组

Question

我希望变量output_format是一个字符串.但是当我运行脚本时,它给了我一个tuple类型并抛出异常.

如果我在Python解释器中运行,它给了我一个预期的字符串.

('--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

脚本源代码:

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

你output_format总是元组,因为你在每个可能的值后面加上一个逗号:

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

删除这些逗号,您的cmd_str遗嘱将再次只包含字符串.

Python元组由这样的逗号组成; 只有在不使用它们时才需要括号会导致含糊不清:

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