title: Update Json value in python
body: <p>I am trying to specify a name for my google spreadsheet api. This is done in the 'title' key value. I have tried with the below but it adds a new key to the existing json. Is there a way to get to the <code>"title": ""</code> and update that value with the <code>new_date</code> item?</p>
  
  <pre><code>prev_date =  datetime.date.today()-datetime.timedelta(1)
  new_date = str(prev_date.isoformat())
  res = {
    "requests": [
      {
        "addSheet": {
          "properties": {
            "title": ""
          }
        }
      }
    ]
  }
  res['title'] =  new_date
  print (res)
  </code></pre>
  
  <p>This is the output:</p>
  
  <pre><code>{'requests': [{'addSheet': {'properties': {'title': ''}}}], 'title': '2016-12-29'}
  </code></pre>
  
  <p>This is what I would like it to be:</p>
  
  <pre><code>{'requests': [{'addSheet': {'properties': {'title': '2016-12-29'}}}]}
  </code></pre>