Episode 0002 of Destroy All Software talks about nil in Ruby. I’m not a Rubyist. I may be someday, but I’m not today, so I just imagined he was talking about None in Python with weird syntax. This episode is really how returning a nil value can lead to exceptions that are miles away from where they actually originated. Gary demonstrates this with a little Rails app, and I found myself nodding along: I see this with some frequency in the Eventbrite codebase, where a domain model’s property is set to None, and at a later point other code tries to call a method on that value.
You can, of course, write your own property descriptor (in Python) that checks for None and raises an exception when that value is set. At least then the error is localized to when it’s really being set to (or returning) None. But what you really want is to avoid the error altogether. Gary shows a couple ways to potentially do that, including inverting the relationship between domain models, and introducing a new model instead of just setting a property on an existing one.