title: Iterate through python string starting at index
body: <p>If I have a very long string (say 100 million characters), is there a way to iterate through the characters using something like <code>for c in str:</code> but starting a certain number of characters in? I'd prefer not to slice the string and use a subset, because I understand that slicing the string will make a copy (very expensive in my case). In other words, can I specify a start point for an iterator over a string?</p>