Python 3000 Status Update (Long!): "Python 3.0 will break backwards compatibility. Totally. We're not even aiming for a specific common subset. (Of course there will be a common subset, probably quite large, but we're not aiming to make it convenient or even possible to write significant programs in this subset. It is merely the set of features that happen to be unchanged from 2.6 to 3.0.)
Python 2.6, on the other hand, will maintain full backwards compatibility with Python 2.5
The recommended development model for a project that needs to support Python 2.6 and 3.0 simultaneously is as follows:
- Start with excellent unit tests, ideally close to full coverage.
- Port the project to Python 2.6.
- Turn on the Py3k warnings mode.
- Test and edit until no warnings remain.
- Use the 2to3 tool to convert this source code to 3.0 syntax. Do not manually edit the output!
- Test the converted source code under 3.0.
- If problems are found, make corrections to the 2.6 version of the source code and go back to step 3.
- When it's time to release, release separate 2.6 and 3.0 tarballs (or whatever archive form you use for releases).
The conversion tool produces high-quality source code, that in many cases is indistinguishable from manually converted code. Still, it is strongly recommended not to start editing the 3.0 source code until you are ready to reduce 2.6 support to pure maintenance (i.e. the moment when you would normally move the 2.6 code to a maintenance branch anyway)."