Python - How to handle bad parameters when creating new instance?

Question

What's the preferred way to handle bad parameters when creating a new instance of a class?

For example, in a card game I might have a card class that can take Club, Heart, Diamond or Spade as the suit when creating a new instance (as well as the value). Do how do I handle someone passing it 'Kitten' as the suit?

I'm new to Python, but know a bit of Objective-C and Swift, and I think in those languages you could make the suit an enum and the compiler might pick up the incorrect value or you could return nil (Ocj-C) or an optional (swift) from the init of the card class.

I understand you cannot return 'nil' in Python (ie not create the instance) so perhaps I should return 'None' for the suit (or whatever member is affected) or even throw an exception? Or should I validate the inputs first?

Answer 1

The class __init__ method should raise a ValueError if it is given something that is incorrect. When in doubt, check out the Zen of Python -- Errors should never pass silently .