Python 3 was released in December 2008. A lot of time has passed since then but still today many developers hesitate to use Python 3. Even popular frameworks like Django are not compatible with Python 3 yet but still rely on Python 2.

Sure, Python 3 has some incompatibilities with Python 2 and some people need to rely on backwards-compatibility. But hasn't Python 3 been around long enough now for most projects to switch or start with Python 3? 

Having two competing versions has so many drawbacks;  two branches need to be maintained, confusion for learners and so on. So why is there so much hesitation throughout the Python community about switching to Python 3? 

Note that I'm no longer updating this answer. I have a much longer Python 3 Q & A on my personal site at http://python-notes.curiousefficiency.org/en/latest/python3/questions_and_answers.html

Previous answer:

(Status update, September 2012)

We (i.e. the Python core developers) predicted when Python 3.0 was released that it would take about 5 years for 3.x to become the "default" choice for new projects over the 2.x series. That prediction is why the planned maintenance period for the 2.7 release is so long.

The original Python 3.0 release also turned out to have some critical issues with poor IO performance that made it effectively unusable for most practical purposes, so it makes more sense to start the timeline from the release of Python 3.1 in late June, 2009. (Those IO performance problems are also the reason why there are no 3.0.z maintenance releases: there's no good reason anyone would want to stick with 3.0 over upgrading to 3.1).

At time of writing (September 2012), that means we're currently a bit over 3 years into the transition process, and that prediction still seems to be on track.

While people typing Python 3 code are most regularly bitten by syntactic changes like print becoming a function, that actually isn't a hassle for library porting because the automated 2to3 conversion tool handles it quite happily.

The biggest problem in practice is actually a semantic one: Python 3 doesn't let you play fast and loose with text encodings the way Python 2 does. This is both its greatest benefit over Python 2, but also the greatest barrier to porting: you have to fix your Unicode handling issues to get a port to work correctly (whereas in 2.x, a lot of that code silently produced incorrect data with non-ASCII inputs, giving the impression of working, especially in environments where non-ASCII data is uncommon).

Even the standard library in Python 3.0 and 3.1 still had Unicode handling issues, making it difficult to port a lot of libraries (especially those related to web services).

3.2 addressed a lot of those problems, providing a much better target for libraries and frameworks like Django. 3.2 also brought the first working version of wsgiref (the main standard used for communication between web servers and web applications written in Python) for 3.x, which was a necessary prerequisite for adoption in the web space.

Key dependencies like NumPy and SciPy have now been ported, installation and dependency management tools like zc.buildout, pip and virtualenv are available for 3.x, the Pyramid 1.3 release supports Python 3.2, the upcoming Django 1.5 release includes experimental Python 3 support, and the 12.0 release of the Twisted networking framework dropped support of Python 2.5 in order to pave the way for creating a Python 3 compatible version.

In addition to progress on Python 3 compatibility libraries and frameworks, the popular JIT-compiled PyPy interpreter implementation is actively working on Python 3 support.

Tools for managing the migration process have also improved markedly. In addition to the 2to3 tool provided as part of CPython (which is now considered best suited for one-time conversions of applications which don't need to maintain support for the 2.x series), there is also python-modernize, which uses the 2to3 infrastructure to target the (large) common subset of Python 2 and Python 3. This tool creates a single code base that will run on both Python 2.6+ and Python 3.2+ with the aid of the six compatibility library. The Python 3.3 release also eliminates one major cause of "noise" when migrating existing Unicode aware applications: Python 3.3 once again supports the 'u' prefix for string literals (it doesn't actually do anything in Python 3 - it's just been restored to avoid inadvertently making migrating to Python 3 harder for users that had already correctly distinguished their text and binary literals in Python 2).

So we're actually pretty happy with how things are progressing - there are still nearly 2 years to go on our original time frame, and the changes are rippling out nicely through the whole Python ecosystem.

Since a lot of projects don't curate their Python Package Index metadata properly, and some projects with less active maintainers have been forked to add Python 3 support, purely automated PyPI scanners still give an overly negative view of the state of the Python 3 library support.

A preferred alternative for obtaining information on the level of Python 3 support on PyPI is http://py3ksupport.appspot.com/

This list is personally curated by Brett Cannon (a long-time Python core developer) to account for incorrect project metadata, Python 3 support which is in source control tools but not yet in an official release, and projects which have more up to date forks or alternatives which support Python 3. In many cases, the libraries that are not yet available on Python 3 are missing key dependencies and/or the lack of Python 3 support in other projects lessens user demand (e.g. once the core Django framework is available on Python 3, related tools like South and django-celery are more likely to add Python 3 support, and the availability of Python 3 support in both Pyramid and Django makes it more likely that Python 3 support will be implemented in other tools like gevent).

The site at http://getpython3.com/ includes some excellent links to books and other resources for Python 3, identifies some key libraries and frameworks that already support Python 3, and also provides some information on how developers can seek financial assistance from the PSF in porting key projects to Python 3.

Another good resource is the community wiki page on factors to consider when choosing a Python version for a new project: http://wiki.python.org/moin/Python2orPython3
