title: how do I create a python list with a negative index
body: <p>I'm new to python and need to create a list with a negative index but have not been successful so far.</p>
  
  <p>I'm using this code:</p>
  
  <pre><code>a = []
  for i in xrange( -20, 0, -1 ):
      a[i] = -(i)
      log.info('a[{i}]={v}'.format(i=i, v=a[i]))
  else:
      log.info('end')
  </code></pre>
  
  <p>and getting the log output as </p>
  
  <pre><code>end
  </code></pre>
  
  <p>Incidentally I'm using a site call quantopian so the log.info is from their infrastructure and just print out the output into a web console.</p>
  
  <p>What am I doing wrong?</p>
  
  <p>Thanks in advance for your help.</p>