title: How can I append serial number into list
body: <p>For example：
  I have a list like<code>['a','b','c','d']</code> and I tend to use python2.7 add the serial number for my list output should like:</p>
  
  <pre><code>[[1,'a'],[2,'b'],[3,'c'],[4,'d']]
  </code></pre>
  
  <p>How can I do this(no other module)?Thanks in advance!</p>
  
  <p>I tend to</p>
  
  <pre><code>a = ['a','b','c','d']
  b = []
  for i,item in enumerate(a,start=1):
  c = b.append([i,item])
  print c
  </code></pre>
  
  <p>but output is <code>None</code></p>