title: Swapping pairs of characters in a string
body: <p>Okay, I'm really new to Python and have no idea how to do this:</p>
  
  <p>I need to take a string, say 'ABAB__AB', convert it to a list, and then take the leading index of the pair I want to move and swap that pair with the __. I think the output should look something like this:</p>
  
  <p><code>move_chars('ABAB__AB', 0)
  '__ABABAB'</code></p>
  
  <p>and another example:</p>
  
  <p><code>move_chars('__ABABAB', 3)
  'BAA__BAB'</code></p>
  
  <p>Honestly have no idea how to do it.</p>