Implement raise errors instead of sys.exit

[tfrederiksen]

Many places we use sys.exit(...) or simply kuk (the lazy, Swedish version) to abort execution.

Nick suggests that it is clearer to raise errors since Python scripts can then continue (if needed).

[zerothi]

Raising errors are (luckily) quite easy.
However, my main problem with raising exceptions is the choice of exceptions to raise.

I try to use a few rules in sisl:

• When input are passed wrong input a ValueError is raised
• When you know that you want to implement a new routine later, but haven't gotten to it. raise NotImplementedError

One should probably use project dependent exceptions to make them easily distinguishable in other projects. A small snippet:

class InelasticaException(Exception):
    pass

def my_routine(*args):
    if args[0] is None:
        raise ValueError('my_routine: first argument cannot be None')
    ...

def ...(...):
     if ...:
        raise InelasticaException('Generic Inelastica exception')

Something like this.

[tfrederiksen]

Thanks Nick. I will try to implement this.