I have seen Garters do that when pressed against the glass (when I was a kid on vacation visiting my grandparents out in Iowa I used to love to love to flip over boards and logs to try to find Garters to catch until my Brother and I flipped over a big piece of plywood and found a nest of Rattlers). The unusual things were that this was a young Ball Python, and it wasn't pressed against the glass. It was kind of acting like a venomous snake rearing for a strike and then sort of dancing, but it was not a venomous snake. I've never seen a Python do that, though it might just be a behavior I haven't seen.