Question

What are the current rules for writing python code that will pass cleanly through 2to3 and what are the practices that seem to be best suited to writing code that will not become mired forever in version 2.

I have read from the SciPy/NumPy forums that “100% test coverage” (unit testing) is important for many people, and I am not sure if that would apply to everybody. Certainly having a reasonable set of unit tests to try your code out with after conversion, seems a sane step.

Are there other things? What are skilled Pythonistas doing if they are writing 2.x code that they hope to have come through “cleanly” in the 2to3 process.

I am looking for specific instances of “[don’t] do this” as well as some more general “best-practices”, but specific instances of “do’s and don’ts” are helpful.

Let’s assume that frameworks, libraries (Django, SciPy/NumPy), and every other C Extension we need gets ported to Python3 eventually, and I’m asking about how you write and maintain the pure python language code that you write yourself.

Update: It’s possible that what I really want is the “style guide” and list of deprecated features that everybody was already staying away from. I cut my teeth on Python 1.5 and moved to 2.0, and then have not really followed much of the 2.5/2.6 era, used them but really my code is more 2.1 era.

Answer 1

I’d say:

• Read the “What’s new for Python 3.0“. Very informative.
  • In particular, if you care about Unicode or text encodings at all, take the time to understand what has changed for 3.x. That’s probably one of the trickier things to change for Python 3.x.
• Get Python 2.6 or 2.7, and run your code with the -3 flag. It will tell you about things in your code that will need changing.
• Before using 3rd-party packages, check to see if they have a Python 3.x version. If not, check the package web site, mailing lists, version control repositories etc to see how actively the package is being developed and whether there is a roadmap towards Python 3.x support.
• Download Python 3.x and try it out! Admittedly, that might not be practical if you care about code that currently depends on packages that don’t yet support Python 3.x (e.g. wxPython or Django).