Hey everyone,
a long time ago we was developing multiplayer sandbox game similar to Minecraft.
Server side is based on Python and Flask web server.
Due to lack of time we have stopped developing.
If anyone likes to continue project developing or just likes to inspire project is avaible on github:
https://github.com/Athelios/OverDark
Demo included.
Greetings from Czech Republic,
Athelios
Very nice ! 
Excellent!
Very cool!
Awesome!
@Athelios is the server here authoritative?
Server is authoritative but not fully.
It controls player inventory content, item amout, item movements, crafting, item drops, block/object placement/destroy.
Other methods like player movement and speed check can be added.
However player movement against collisions is client-trusted. Server has no collision and physics engine.