Hi guys,
thought I’d share my infinite wisdom 
Kept on getting anvil.server.RuntimeUnavailableError: Data payload too big - please use Media objects to transfer large amounts of data.
Without any printout of the line where the error was occuring.
Turns out I was in a background task and had a print in it that was printing out my large dataset (stupid of me, but didnt think much of it). That’s what caused the problem 