I have been working with Python 3 for over a year. I used it in
writing my book "Bioinformatics Programming Using Python" (http://oreilly.com/catalog/9780596154509
). I didn't see any point in teaching an incompatible earlier version
of a language in transition. In preparing the book and its examples I
explored a large number of Python modules in some depth and
encountered most of the differences between the language and libraries
of Python 2 and Python 3. The change was a bit awkward for a while,
and there were some surprises, but in the end I have found nothing in
Python 3 for which I would prefer Python 2's version.
Removal of old-style classes is a big win. Having print as a function
provides a tremendous amount of flexibility. I use the sep and end
keywords all the time. There is no reason for print to be a statement,
and it was an awkward inconsistency in a language that leans towards
functional styles. Likewise the elimination of cmp, while shocking,
leads to much simpler comparison arguments to sort, since all the
function does is return a key; then, sort uses __lt__ (I think) so it
automatically uses each class's definition of that. The weird objects
returned from things like sorted, dict.keys/values/items, and so on
are values that in practice are used primarily in iterations; you can
always turn the result into a list, though I have to admit that while
developing and debugging I trip trying to pick out a specific element
from one of these using indexing (typically [0]); I've learned to
think of them as generators, even though they aren't. The
rearrangements and name changes in the libraries are quite helpful. I
could go on, but basically the language and library changes are on the
whole large improvements with little, if any, downside.
Conversion of old code is greatly facilitied by the 2to3 tool that
comes with Python 3. The big issue in moving from 2 to 3 is the
external libraries and development tools you use. Different IDEs have
released versions that support Python 3 at different times. (I believe
Wing was the first.) If you use numpy, for example, or one of the many
libraries that require it, you are stuck. Possibly some important
facilities will never be ported to Python 3, but probably most active
projects will eventually produce a Python 3 version -- for example,
according to its web page, a Python 3 version of PIL is on the way. I
was able to cover all the topics in my book using only Python library
modules, something I felt would be best for readers -- I used
libraries such as elementree, sqlite3, and tkinter. The only
disappointment was that I couldn't include a chapter on BioPython,
since there is no Python 3 version.
By now, many large facilities support both Python 2 and Python 3. I am
currently building a complex GUI/Visualization application based on
the Python 3 version of PyQt4 and Wing IDE and am delighted with all
of it. It may well be that some very important large
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