Python 2 and Python 3 dual development

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In my experience, it depends on the kind of project.

If it is a library or very self contained application, a common choice is develop in Python 2.7 avoiding constructs deprecated in Python 3.x as much as possible and resort to automated tests to identify holes left by py2to3 that you will have to fix manually.

On the other side, for real life applications, be prepared to constantly stumble upon libraries that are not ported to py3k yet (sometimes important ones). Most of the time you will have no choice but port the library to Python 3, so if you can afford that, go for it. Usually I can’t, that is why I’m not supporting Python 3 for this kind of project (but I struggle to write code that will be easier to port when opportune).

For unicode handling, I found this PyCon 2012 video very informative. The advice is good for both Python 2.x and 3.x: treat every string coming from outside as bytes and convert to unicode as soon as possible and output strings converting to bytes as late as possible. There is another very informative video about date/time handling.

[update]

This is an old answer. As of today (2019) there is no good rationale to start a project using Python 2.x and there are several compelling reasons to port older projects to Python 3.7+ and abandon support for Python 2.x.

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