I noticed that. The fact that the `in` if-statement worked if it was a string too. Python is neat. 

My objection was that it didn't feel tight. 

1. we make a decision on what `site_url` is and then we allow manipulation and inquiry into it. Like, I say my name is "Peter" and then you ask if it's a list. No, that's definitely my name :)
2. The `in` operation on strings is maybe not what you need. What if you want to allow "github.io" but the current request URL is "peterbe.github.io" then the if-statement will say yes. 

I didn't know `isinstance` was looked down on. I'm not a huge fan of it but it works well. 

@Osmose What do you think? Ignoring the implementation details for a moment, what do you expect `settings.SITE_URL` to be allowed to be? ...apart from just a plain string.