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.
Some of the decisions made in regards to unicode and byte untilization 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.
A lot of the documentation out there on using WSGI leaves out those details as it was written before WSGI was updated to Python 3. While the API for Werkzeug and Flask on Python 2.x should not change much we cannot guarantee that this won’t happen on Python 3.
Python 3 currently has less than 1% of the users of Python 2 going by PyPI download stats. As a result many of the problems you will encounter are probably hard to search for on the internet if they are Python 3 specific.
The majority of the Flask extensions, all of the documentation and the vast majority of the PyPI provided libraries do not support Python 3 yet. Even if you start your project with knowing that all you will need is supported by Python 3 you don’t know what happens six months from now. If you are adventurous you can start porting libraries on your own, but that is nothing for the faint of heart.
Unless you are already familiar with the differences in the versions we recommend sticking to current versions of Python until the ecosystem caught up.
The majority of the upgrade pain is in the lower-level libararies 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.