title: Pandas Coalesce Multiple Columns, NaN
body: <p>I want to <code>coalesce</code> 4 columns using pandas. I've tried this:</p>
  
  <p><code>final['join_key'] = final['book'].astype('str') + final['bdr'] + final['cusip'].fillna(final['isin']).fillna(final['Deal'].astype('str')).fillna(final['Id'])</code></p>
  
  <p>When I use this it returns:</p>
  
  <pre><code>+-------+--------+-------+------+------+------------+------------------+
  | book  |  bdr   | cusip | isin | Deal |     Id     |     join_key     |
  +-------+--------+-------+------+------+------------+------------------+
  | 17236 | ETFROS |       |      |      | 8012398421 | 17236.0ETFROSnan |
  +-------+--------+-------+------+------+------------+------------------+
  </code></pre>
  
  <p>The field <code>Id</code> is not properly appending to my <code>join_key</code> field.</p>
  
  <p>Any help would be appreciated, thanks.</p>
  
  <p><strong>Update</strong>:</p>
  
  <pre><code>+------------+------+------+-----------+--------------+------+------------+----------------------------+
  |  endOfDay  | book | bdr  |   cusip   |     isin     | Deal |     Id     |          join_key          |
  +------------+------+------+-----------+--------------+------+------------+----------------------------+
  | 31/10/2019 |   15 | ITOR | 371494AM7 | US371494AM77 |  161 | 8013210731 | 20191031|15|ITOR|371494AM7 |
  | 31/10/2019 |   15 | ITOR |           |              |      | 8011898573 | 20191031|15|ITOR|          |
  | 31/10/2019 |   15 | ITOR |           |              |      | 8011898742 | 20191031|15|ITOR|          |
  | 31/10/2019 |   15 | ITOR |           |              |      | 8011899418 | 20191031|15|ITOR|          |
  +------------+------+------+-----------+--------------+------+------------+----------------------------+
  
  df['join_key'] = ("20191031|" + df['book'].astype('str') + "|" + df['bdr'] + "|" + df[['cusip', 'isin', 'Deal', 'id']].bfill(1)['cusip'].astype(str))
  </code></pre>
  
  <p>For some reason this code isnt picking up <code>Id</code> as part of the key.</p>