Yes, I have checked that the Project Variable is defined - it appears in the Project Variables and in the Variables section of the Python recipe that I am trying to run. However, when I attempt to build the dataset, it appears that it is trying to build against the partition "${current_month}" rather than "2019-01", which is what the variable points to:
compute_partitioned_output (${current_month})
I also get the following error after launching the dataset build:
Error in Python process: At line 8: : No JSON object could be decoded