title: ffmpeg in Python subprocess - Unable to find a suitable output format for 'pipe:'
body: <p>Trying to burn subs into video with ffmpeg via Python.  Works fine in the command line, but when calling from Python subprocess with:</p>
  
  <pre><code>p = subprocess.Popen('cd ~/Downloads/yt/; ffmpeg -i ./{video} -vf subtitles=./{subtitles} {out}.mp4'.format(video=vid.replace(' ', '\ '), subtitles=subs, out='out.mp4'), shell=True)
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>Unable to find a suitable output format for 'pipe:'
  </code></pre>
  
  <p>Full traceback:</p>
  
  <pre><code>'ffmpeg version 2.7.2 Copyright (c) 2000-2015 the FFmpeg developers
    built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
    configuration: --prefix=/usr/local/Cellar/ffmpeg/2.7.2_1 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfreetype --enable-libvpx --enable-libass --enable-libfdk-aac --enable-nonfree --enable-vda
    libavutil      54. 27.100 / 54. 27.100
    libavcodec     56. 41.100 / 56. 41.100
    libavformat    56. 36.100 / 56. 36.100
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 16.101 /  5. 16.101
    libavresample   2.  1.  0 /  2.  1.  0
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  2.100 /  1.  2.100
    libpostproc    53.  3.100 / 53.  3.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from './OnHub - a router for the new way to Wi-Fi-HNnfHP7VDP8.mp4':
    Metadata:
      major_brand     : isom
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf56.36.100
    Duration: 00:00:53.94, start: 0.000000, bitrate: 2092 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 1961 kb/s, 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc (default)
      Metadata:
        handler_name    : VideoHandler
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 125 kb/s (default)
      Metadata:
        handler_name    : SoundHandler
  [NULL @ 0x7fc07b077600] Unable to find a suitable output format for 'pipe:'
  pipe:: Invalid argument'
  </code></pre>