Flask is a web framework for Python. It is a very lightweight framework, meaning it covers only the basics you need to get a web server running. It's an excellent introduction to server side development if you're familiar with Python.
(jeffknupp.com)
Understanding exactly what a web framework is will be important before you write a single line of server side code. This technical overview of what a web framework is and how it works in common Python frameworks will cover HTTP requests, routing, and even templates.
(flask.pocoo.org)
This is a quick tutorial on how to install Flask using the shell, pip, and virtualenv. If you don't have Flask installed yet, follow the instructions here.
(flask.pocoo.org)
Build a basic web application using this documentation. This is one of the best written getting started guides and it should give you the basics you'll need for Flask. Go through the whole thing.
Some of these tutorials will mention hooking up your Flask application to a database like SQLite, Postgres, or some other database. If you aren't familiar with these technologies, don't worry, you can go learn about how these work in another track later on.
(learn.rmotr.com)
This is a comprehensive and free course on how to build web applications with Flask and Python. A combination of videos and code examples will give you a great general overview of how to use the Python web framework.
(jinja.pocoo.org)
You had some exposure to templating with Jinja in the previous tutorials. Now go through the documentation of Jinja to learn its full power as a templating engine in Flask.
(exploreflask.com)
This online book covers some best practices when creating a Flask application. Everything from file structure, deployment, and naming conventions is covered here.