title: sort function in python 3
body: <p>I am a beginner in python. i sorted a list of string values but
  '8,13' '8,14'.... and '9,11' '9,12'.... values came to last. what causes this error?
  I typed this in terminal.</p>
  
  <pre><code>a = ['13,7', '14,7', '15,7', '12,8', '13,8', '14,8', '15,8', '16,8', 
       '11,9', '12,9', '13,9', '16,9', '10,10', '11,10', '12,10', 
       '16,10', '9,11', '10,11', '11,11', '16,11', '9,12', '10,12', 
       '16,12', '8,13', '9,13', '10,13', '15,13', '16,13', '8,14', 
       '9,14', '15,14', '16,14', '17,14', '8,15', '9,15', '14,15', 
       '16,15', '17,15', '18,15', '19,15', '8,16', '9,16', '12,16', 
       '13,16', '17,16', '18,16', '19,16', '8,17', '9,17', '10,17', 
       '11,17', '12,17', '9,18', '10,18', '11,18']
  
  a.sort()
  
  a
  #['10,10', '10,11', '10,12', '10,13', '10,17', '10,18', '11,10', 
  # '11,11', '11,17', '11,18', '11,9', '12,10', '12,16', '12,17', 
  # '12,8', '12,9', '13,16', '13,7', '13,8', '13,9', '14,15', '14,7', 
  # '14,8', '15,13', '15,14', '15,7', '15,8', '16,10', '16,11', '16,12', 
  # '16,13', '16,14', '16,15', '16,8', '16,9', '17,14', '17,15', 
  # '17,16', '18,15', '18,16', '19,15', '19,16', '8,13', '8,14', '8,15', 
  # '8,16', '8,17', '9,11', '9,12', '9,13', '9,14', '9,15', '9,16', 
  # '9,17', '9,18']
  </code></pre>