Question

So with the final releases of Python 3.0 (and now 3.1), a lot of people are facing the worry of how to upgrade without losing half their codebase due to backwards incompatibility.

What are people’s best tips for avoiding the many pitfalls that will almost-inevitably result from switching to the next-generation of python?

Probably a good place to start is “use 2to3 to convert your python 2.x code to 3.x” 🙂

Answer 1

First, this question is very similar to How are you planning on handling the migration to Python 3?. Check the answers there.

There is also a section in the Python Wiki about porting applications to Python 3.x

The Release Notes for python 3.0 contains a section about porting. I’m quoting the tips there:

1. (Prerequisite:) Start with excellent test coverage.
2. Port to Python 2.6. This should be no more work than the average por
   from Python 2.x to Python 2.(x+1).
   Make sure all your tests pass.
3. (Still using 2.6:) Turn on the -3 command line switch. This enables warnings about features that will be
   removed (or change) in 3.0. Run your
   test suite again, and fix code that
   you get warnings about until there are
   no warnings left, and all your tests
   still pass.
4. Run the 2to3 source-to-source translator over your source code tree.
   (See 2to3 – Automated Python 2 to 3
   code translation for more on this
   tool.) Run the result of the
   translation under Python 3.0. Manually
   fix up any remaining issues, fixing
   problems until all tests pass again.

It is not recommended to try to write
source code that runs unchanged under
both Python 2.6 and 3.0; you’d have to
use a very contorted coding style,
e.g. avoiding print statements,
metaclasses, and much more. If you are
maintaining a library that needs to
support both Python 2.6 and Python
3.0, the best approach is to modify step 3 above by editing the 2.6
version of the source code and running
the 2to3 translator again, rather than
editing the 3.0 version of the source
code.