When I started my current project two and half years ago I stuck with Python 2 because I was concerned about library support even though the major libraries I was using (Pyramid and SQLAlchemy) already supported Python 3.
While I think that was the right decision at the time, for me the balance shifted about 6 months. Needing access to the much improved multiprocessing library in Python 3 I ported my application to polyglot with the help of the python-future library. It really wasn't very difficult.
This was a modern codebase of about 20K LOC with reasonably high test coverage. For much larger or older applications then not porting is probably the right choice. But for new applications I think Python 3 is a fairly good bet now. Even if only 15-20% of the Python ecosystem have switched then that's still a significant enough userbase for most libraries to have had their Python 3 bugs shaken out.
While I think that was the right decision at the time, for me the balance shifted about 6 months. Needing access to the much improved multiprocessing library in Python 3 I ported my application to polyglot with the help of the python-future library. It really wasn't very difficult.
This was a modern codebase of about 20K LOC with reasonably high test coverage. For much larger or older applications then not porting is probably the right choice. But for new applications I think Python 3 is a fairly good bet now. Even if only 15-20% of the Python ecosystem have switched then that's still a significant enough userbase for most libraries to have had their Python 3 bugs shaken out.