Python 3 Support¶
Flask and all of its dependencies support Python 3 so you can in theory start working on it already. There are however a few things you should be aware of before you start using Python 3 for your next project.
If you want to use Flask with Python 3 you will need to use Python 3.3 or higher. 3.2 and older are not supported.
In addition to that you need to use the latest and greatest versions of itsdangerous, Jinja2 and Werkzeug. Flask 0.10 and Werkzeug 0.9 were the first versions to introduce Python 3 support.
Some of the decisions made in regards to unicode and byte utilization on Python 3 make it hard to write low level code. This mainly affects WSGI middlewares and interacting with the WSGI provided information. Werkzeug wraps all that information in high-level helpers but some of those were specifically added for the Python 3 support and are quite new.
Unless you require absolute compatibility, you should be fine with Python 3 nowadays. Most libraries and Flask extensions have been ported by now and using Flask with Python 3 is generally a smooth ride. However, keep in mind that most libraries (including Werkzeug and Flask) might not quite as stable on Python 3 yet. You might therefore sometimes run into bugs that are usually encoding-related.
The majority of the upgrade pain is in the lower-level libraries like Flask and Werkzeug and not in the actual high-level application code. For instance all of the Flask examples that are in the Flask repository work out of the box on both 2.x and 3.x and did not require a single line of code changed.