Fail fast

[KosRud]

https://github.com/cl0ne/euro-diffusion/blob/0c6495853144868453839c0651b6c53136e88ba4/euro_diffusion/simulator.py#L28-L31

https://github.com/cl0ne/euro-diffusion/blob/0c6495853144868453839c0651b6c53136e88ba4/euro_diffusion/simulator.py#L63-L65

In case when list countries is empty it is already clear that something went wrong. There is no point in continuing execution with corrupted data

[cl0ne]

It'll just return an empty list in that case.

[KosRud]

https://github.com/cl0ne/euro-diffusion/blob/0c6495853144868453839c0651b6c53136e88ba4/main.py#L19-L20

the countries list is never supposed to be empty because if it is, the Simulator is not created

[cl0ne]

I know that and I don't see any reason why simulator cannot return empty list for empty input even if the client code won't cause this situation.