The main differences between Python 3 and Python 2

It has already been stated that Python 3 breaks backward compatibility with Python 2 on a syntax level. Still, it is not a complete redesign. Also, it does not cause every Python module written for some 2.x release to stop working under Python 3. It is possible to write completely cross-compatible code that will run on both major releases without additional tools or techniques, but usually it is possible only for simple applications.