When I was a kid I remember one "fast" Sunday in particular (once a month you donate the money you would have spent on food to the poor - you "fast" and get all sorts of spiritual benefits). I was probably 4 or 5 and some big person asked me if I had fasted. I said, "yes! I only ate one banana!" There was lots of laughter and I was embarrassed that I hadn't been 100% faithful to the idea of fasting.
Yesterday I went to two parties. I was good! I only ate 3 creampuffs!
I think the only way to make sure there's something at the party that you can eat is to take something that you can eat. Otherwise there's no guarantee.
No guarantee, but most people serve both proteins and carbs, and there's plenty of water next to that soda.
I took a fruit salad. Fresh cherries (picked yesterday morning even) and strawberries and pineapple in a fat-free, low-sugar cream sauce. Yum. I still ate too much, but it wasn't a bad day.