title: Exploding my pandas columns does not work anymore
body: <p>I need to explode some columns in the below dataset:</p>
  <pre><code>Date    Borr    Year    Acc Grade
  2020-06-30  borrower1   2001.0  ['acc1', 'acc2', 'acc3',...]    [3.6, 5.2, 3.8,...]
  2020-06-30  borrower1   2001.0  ['acc1', 'acc2', 'acc3',...]    [3.6, 5.2, 3.8,...]
  2020-06-30  borrower2   2005.0  ['acc23', 'acc21']  [6.6, 5.2]
  </code></pre>
  <p>I did as follows</p>
  <pre><code>new_df=df.apply(pd.Series.explode).reset_index()
  </code></pre>
  <p>but it still contains lists. For each account I have a grade.
  I have always used explode as in the example above (and to be honest it worked until a few days ago).
  Has anyone idea on the reason why it does not work anymore?
  I have already looked at other questions (e.g., <a href="https://stackoverflow.com/questions/62801562/pandas-explode-multiple-columns">Pandas explode multiple columns</a>), so please do not treat this question as a duplicate, as it is not.</p>