Resources

repl.it

repl.it is a web-based development environment that supports Python along with many other languages. The IDE is pretty slick and you don't even have to sign up to give it a try: Python3 environment on repl.it. This is probably the easiest place to get started with Python3.

pythonAnywhere

pythonAnywhere is another web-based development environment where you can code, compile, and run python programs. It will even host your web apps. The beginner plan is completely free. Paid plans support Jupyter Notebooks.

Jupyter Notebook

A jupyter notebook is something altogether different: it is an environment where you can integrate code, data, and analysis all in one file. Documentation is here.