title: How to flat a pandas series of json string into a dataframe
body: <p>I have a pandas dataframe</p>
  
  <pre><code>            date_hour                                            content
  0 2016-10-17 00:00:00  [{"81": 0.0, "82": 0.0, "83": 0.0}]
  1 2016-10-17 01:00:00  [{"81": 0.0, "82": 0.0, "83": 0.0}]
  </code></pre>
  
  <p>I want to flat the df.content into a dataframe like this</p>
  
  <pre><code>                        81     82  83  
  2016-10-17 00:00:00      0      0   0
  2016-10-17 01:00:00      0      0   0
  </code></pre>
  
  <p>how can I achieve this?</p>
  
  <p>I tried:</p>
  
  <pre><code># work for one item, though I can concat them, but it's slow(I have each json of 7k k/v pairs), took 2.5s for each
  pd.read_json(df.head(1).content.item(), orient='records')
  </code></pre>