title: How to fetch JSON argument in Tornado's POST handler?
body: <p>From client I generate post request and in body I have simlpe JSON like</p>
  
  <pre><code>{
      "action": "copy_all",
      "id": "aabbababab",
      "factory_id": 12297829382473034000
  }
  </code></pre>
  
  <p>How to fetch this argument this in handler (post function) ?
  I know how to parse when I send like key/value pair and get argument by key, but here JSON is one argument.</p>