JavaScript (ie. Node) for custom code would be much more useful than Python. (According to Stack Overflow, JS is the most commonly used programming language.)

Python is a nice extra option, but about 99% of our users (and 100% of our staff) know JS, while it's closer to 0% for Python. So the custom code being Python-only at this point makes it much, much less useful than it otherwise would be (and custom code is HUGELY useful).