title: Fastest way to update a character in a string in Python
body: <p>I have to replace every '%' character with '%%' in a string and as string are immutable i am doing <code>list(string)</code> and then replace every '%' with '%%'.
  But string can be huge(len(string)>2000) and as a result list can be huge that can slow down so i want to know the fastest way to do that in python 2.7</p>