Before we start digging into Flask, there's one question we should answer. What Python version should be used at this point with Flask, since it supports both?

We're now in 2017, and as we've seen in the previous chapter, Python 3 has made some incredible progress. Packages that don't support Python 3 are now less common. Unless you're building something very specific, you should not have any problem with Python 3.

And building microservices means each app will run in isolation, so it would be entirely imaginable to run some in Python 2 and some in Python 3 depending on your constraints. You can even using PyPy.

Despite the initial pushbacks the Flask creator had on some of the Python 3 language decisions, the documentation explicitly says at this point that new projects should start using Python 3; refer to http://flask.pocoo.org/docs/latest/python3/#python3-support.

Since Flask is not using any new bleeding-edge Python 3 language features, your code will probably be able to run in Python 2 and 3 anyway. In the worst case, you can use a tool like Six (http://pythonhosted.org/six/) to make your code compatible with both versions if you need to.

The general advice is to use Python 3 unless you have some constraints that require Python 2. Python 2 will not be supported anymore after 2020; see https://pythonclock.org/.