Iterating through lists, in and not in, list comprehensions with conditions

The most common thing to do with a list is to process each item in turn. A for loop over the list gives you each element directly, one per iteration: for price in prices: total += price. The loop variable is simply bound to the next element each time; it is not a position. If you also need positions, loop over range(len(prices)) or use enumerate(prices), which yields pairs of index and value. When you need to change items in place, use the index form (prices[i] = prices[i] * 2), because assigning to the loop variable only rebinds that name and does not modify the list.