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:
- (Prerequisite:) Start with excellent test coverage.
- 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.
- (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.
- 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.