.. _cporting-howto:

Due to time constraints, this document does not exhaustively cover

the very extensive changes to the library.

For porting existing Python 2.5 or 2.6 source code to Python 3.0, the

best strategy is the following:

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

1. Port to Python 2.6. This should be no more work than the average

port from Python 2.x to Python 2.(x+1). Make sure all your tests

pass.

2. (Still using 2.6:) Turn on the :option:`-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.

3. Run the ``2to3`` source-to-source translator over your source code

tree. (See :ref:`2to3-reference` 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 :keyword:`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.