title: rotate pandas DataFrame with rows of json to plain DataFrame
body: <p>I have a pandas dataframe that looks like this</p>
  <pre class="lang-py prettyprint-override"><code>df_in = pd.DataFrame(data = {'another_col': ['a', 'x', '4'], 'json': [
      [{&quot;Key&quot;:&quot;firstkey&quot;, &quot;Value&quot;: 1.4}, {&quot;Key&quot;: &quot;secondkey&quot;, &quot;Value&quot;: 6}],
      [{&quot;Key&quot;:&quot;firstkey&quot;, &quot;Value&quot;: 5.4}, {&quot;Key&quot;: &quot;secondkey&quot;, &quot;Value&quot;: 11}],
      [{&quot;Key&quot;:&quot;firstkey&quot;, &quot;Value&quot;: 1.6}, {&quot;Key&quot;: &quot;secondkey&quot;, &quot;Value&quot;: 9}]]}
  )
  </code></pre>
  <p>which when printed looks like</p>
  <pre><code>  another_col                                               json
  0           a  [{'Key': 'firstkey', 'Value': 1.4}, {'Key': 's...
  1           x  [{'Key': 'firstkey', 'Value': 5.4}, {'Key': 's...
  2           4  [{'Key': 'firstkey', 'Value': 1.6}, {'Key': 's...
  </code></pre>
  <p>I need to transform it and parse each row of json into columns. I want the resulting dataframe to look like</p>
  <pre><code>  another_col  firstkey  secondkey
  0           a       1.4          6
  1           x       5.4         11
  2           4       1.6          9
  </code></pre>
  <p>How do I do this? I have been trying with <code>pd.json_normalize</code> with no success.
  A secondary concern is speed... I have to apply this on ~5mm rows...but first let's get it working. :-)</p>