title: Why can't download subtitle with pure python?
body: <p>Download the subtitle with youtube-dl command:  </p>
  
  <pre><code>url="https://www.youtube.com/watch?v=Ix8xPfKDxNg"
  youtube-dl --write-auto-sub --skip-download --sub-lang en  $url
  [info] Writing video subtitles to: Speak English Confidently &amp; Clearly with Ellen-Ix8xPfKDxNg.en.vtt
  </code></pre>
  
  <p>Now i want to do the same task with pure python code.</p>
  
  <pre><code>from __future__ import unicode_literals
  import youtube_dl
  url="https://www.youtube.com/watch?v=Ix8xPfKDxNg"
  options = {
          'writeautomaticsub': True,
          'subtitleslangs': ['en'],
          'skip_download': True,
          'subtitleslangs': 'en'
  }
  with youtube_dl.YoutubeDL(options) as ydl:
      ydl.download([url])
  </code></pre>
  
  <p>The error info:</p>
  
  <pre><code>WARNING: e subtitles not available for Ix8xPfKDxNg
  WARNING: n subtitles not available for Ix8xPfKDxNg
  </code></pre>
  
  <p>Why can't replace the bash command</p>
  
  <pre><code>youtube-dl --write-auto-sub --skip-download --sub-lang en  $url
  </code></pre>
  
  <p>with pure python code?</p>