This is a rewrite of the Python 2/3 porting HOWTO to describe how to make code be compatible with both Python 2 & 3. To see it rendered for easier reading, see https://gist.github.com/brettcannon/91ff264ae549315706f6

Why you have removed many sections: about subclassing from object, using io classes, comparison of bytes with unicode objects, the use of warnings and -b and -3 options and other subtle details?

I removed a bunch of sections for two reasons. One is they are redundant. If you follow the HOWTO and actually read What's New then you will get a bunch of those same details anyway. The tools will also handle the details for you and so you really don't have to worry about them (and they document them already). And Pylint will warn you when you have not taken care of them. So having a fourth place listing the fact you need to subclass object is simply a waste.

Second is accidental FUD. If you have a huge list of things to watch out for then you are going to end up scaring people off by making them feel intimidated at the work ahead of them. But if you reframe the whole thing as "just follow this TODO list and you will be fine" you can make people realize that most things are simply not a big deal.

But the point about telling people to run with -bb and -3 and a couple of minor comments are valid and I will toss some of them back in.

I addressed Serhiy's comments and added in some bits that I initially took off (-bb, -3, str.__mod__, io.open). Also clarified a couple of things.

New changeset 55b94462ca7f by Brett Cannon in branch '3.4':
Issue #22914: Update the Python 2/3 porting HOWTO to describe a more
https://hg.python.org/cpython/rev/55b94462ca7f