it's an old post but I want to show my discoveries that I did right now.

When opening a file in text mode "r+", Python will use line buffering when reading. However if you open in binary mode "br+", Python will allow you use the "buffering" parameter and you can put "0" to turn it off. So far, in my small tests "f.tell()" has given me correct results, but you will need to use "str.decode()".