title: How to find the position of an item of a list in python?
body: <p>I have two lists. The first list has a specific length and the second list has unstable length. I want to assign the items from two lists.
  I use <code>colour_index = sel_pos%(len(colours))</code>. The sel_pos is the item's position from the unstable list.
  How to find the position every time?</p>