I’ve already made one module Python3-compatible. I decided to bite the bullet and make my not-really-released winsys module completely Python3. I’d tried the 2to3.py approach but it left just enough false positives & negatives (sorry, don’t remember the details) that I decided to go by hand, including translating the unit tests from nose format to the updated unittest format.

Although I was reasonably clued-in on the changes from 2 to 3, I was caught out by a few things, which I list here in case they help anyone else:

• __nonzero__ is now __bool__: I knew this perfectly well, but it took me a while to find out why my tests were failing. 2to3 would have picked this up straightaway.
• __hash__ isn’t inherited if __eq__ is defined: this one surprised me and took a bit of research. (All right; I actually read the docs for __hash__).
• bytes vs str: well, obviously you knew this, but it’s bitten me in some slightly unexpected places when interfacing via pywin32. The error message speaks of expecting a buffer object, which threw me slightly.
• assertTrue doesn’t bool its param: UPDATE - of course this isn’t true at all; I was confusing this and the __bool__/__nonzero__ issue above. Thanks to Marius Gedminas for pointing this out.
• pywin32 generates tz-aware datetime objects: for a long time, pywin32 rolled its own datetime type (as Python had none). Last year, Mark H canvassed the python-win32 list about committing to the Python datetime type. And now it’s there. But it’s tz-aware. And the core Python type isn’t. Fortunately, pywin32 has for a while shipped with the win32timezone module which solves the issue.

I’m sure there’ll be more which I’ll add here as they come along.