title: Changing first item in nested list using list comprehension
body: <p>Let's say I have the following list of lists:</p>
  
  <pre><code>[['Hello im an item', 1234], ['I enjoy python', 'hello'], ['I only have one item']]
  </code></pre>
  
  <p>I want to replace the spaces with underscores in the first item in each inner list so it looks like this:</p>
  
  <pre><code>[['Hello_im_an_item', 1234], ['I_enjoy_python', 'hello'], ['I_only_have_one_item']]
  </code></pre>
  
  <p>I have tried the following with no success:</p>
  
  <pre><code>formatted = [first[0].replace(' ', '_') for first in formatted]
  </code></pre>
  
  <p>formatted returns a list of only the first items, but with the correct formatting.
  Does anyone have any idea how this can be accomplished?</p>