what is the easiest way to transfer codes from Python 2.0 to 3.0.

Answer 1

I found something amazing to do this: the "future" module. Basically, you can use it in python 2 to use the python 3 syntax. With it you can prepare slowly yours scripts and then use python 3.
see : http://sebastianraschka.com/Articles/2014_python_2_3_key_diff.html#the-future-module

Answer 2

knowing the syntax and features of both. Enough to know whether or not the code would work on py3.