title: Switch every pair of characters in a string
body: <p>For example, having the string:</p>
  
  <pre><code>abcdefghijklmnopqrstuvwxyz
  </code></pre>
  
  <p>should result in something like this:</p>
  
  <pre><code>badcfehgjilknmporqtsvuxwzy
  </code></pre>
  
  <p>How do I even go about it?</p>
  
  <p>I thought of something not very efficient, such as:</p>
  
  <pre><code>s = str(range(ord('a'), ord('z') + 1))
  new_s = ''
  for i in xrange(len(s)):
      if i != 0 and i % 2 == 0:
          new_s += '_' + s[i]
      else:
          new_s += s[i]
  # Now it should result in a string such as 'ab_cd_ef_...wx_yz'
  l = new_s.split('_')
  for i in xrange(len(l)):
      l[i] = l[i][::-1]
  result = str(l)
  </code></pre>
  
  <p>Is there any better way ? Some way that is more efficient or more general so I could also go about it with 3 letters more easily ?</p>