title: insert value string into list where list in each cell of column
body: <p>My input:</p>
  <pre><code>df=(pd.DataFrame({'items':['foo','bar','xyz'],
                    'path':['c/folderOne/folderTwo/folderThree','c/folder1/folder2/folder3','c/folderO/folderT/folderTh'],
  }))
  </code></pre>
  <p>I trying insert in column <code>path</code> some &quot;fix&quot; after <code>'c/...</code> I want add new substring such as <code>NEWPATH</code> into list and re-join again, but I ran <em>unexpected</em> problem.<br />
  That my code:</p>
  <pre><code>df['path'] = df['path'].map(lambda x: x.split(&quot;/&quot;))
  df['path'] = df['path'].map(lambda x: x.insert(1,'NEWPATH'))
  df['path'] = df['path'].map(lambda x: x.join('/'))
  </code></pre>
  <p>Code return <code>None</code> in column <code>path</code>, when I <strong>expected</strong> something like this, after second line of my code:<br />
  in column <code>df['path'] : ['c','NEWPATH','folderOne'....]</code> etc. for each row(cell) of my column.</p>