title: How to append a row of integers into a single list across multiple columns in pandas
body: <p>Lets say I have the follow dataframe</p>
  
  <pre><code>    col_1    col_2
  0    1        2   
  1    3        4
  </code></pre>
  
  <p>and I want to create a third column that is a <em>list of integers</em></p>
  
  <pre><code>    col_1    col_2    col_3
  0     1        2      [1,2]
  1     3        4      [3,4]
  </code></pre>
  
  <p>I tried several messy approaches where I convert the values into strings, concatenate them, and the turn it into a list of integers like this:</p>
  
  <pre><code>df['col_3'] = df[['col_1', 'col_2']].apply(lambda x: ','.join(str(x)), axis=1)
  </code></pre>
  
  <p>But it was super messy and super slow and did not return what I was looking for. Is there a better way of doing this?</p>