Repl.it is an online code sandbox and IDE.
It supports many languages, and makes demoing and prototyping code a piece of cake.

It has some caveats though.

PyGame

There’s no PyGame environment available on repl.it, making any PyGame project impossible.

Turtle

The Turtle environment on repl.it is Python 2.7 - can’t teach Python 3.6 syntax here!

Showing Images

For the UNSW CompClub 2019 Summer Workshop, the Security workshop introduced students to LSB steganography.
Sometimes repl.it would show the result image created by PIL, but other times it wouldn’t…
I ended up asking the students to use the local Python instance on their machines instead of working through repl.it

Jinja2 x Flask

https://aryaboudaie.com/python/technical/educational/web/flask/2018/10/17/flask.html

“Note - for using templates on repl.it, you need to have at least two python files in your workspace because of a bug. The second Python file can be completely empty, but that’s why I have the dummy.py file in the REPL.”

Uhm okay…

Overall, I’m really pleased with what repl.it has to offer.
Other sites like trinket.io are also promising, but their pricing model makes it hard to use in an educational environment