Hardly a beginner at programming, but I also struggled with this when I picked up Python recently. The documentation doesn't make it obvious when functions have

Hardly a beginner at programming, but I also struggled with this when I picked up Python recently. The documentation doesn't make it obvious when functions have this sort of side effect, so like your students I play with the REPL to establish exactly what a function will do. Even then an unexpected side effect was the source of the single most perplexing bug in my program. I imagine that in complex programs this philosophy of sometimes-mutability-sometimes-not must lie at the root of a huge amount of programming error.