title: Why list indices must be integers, not tuple?
body: <p>I have this simple program:</p>
  
  <pre><code>x = {}
  x[1,2] = 3
  print x
  print x[1,2]
  </code></pre>
  
  <p>It works fine. The fist <code>print</code> generates <code>{(1,2):3}</code> and the second one generates <code>3</code>.</p>
  
  <p>But in my "big" program I seems to do the same but get a <code>list indices must be integers, not tuple</code> error. What this error message can mean and how I can resolve this problem?</p>