I adore list comprehensions, but perhaps for this reason, I generally only use them for operations so simple as to be trivially correct or incorrect without any

I adore list comprehensions, but perhaps for this reason, I generally only use them for operations so simple as to be trivially correct or incorrect without any debugging. Dict comprehensions are just a little muddier, even for simple things, but a single line of debug output after the comprehension should confirm whether or not you're getting the structure you expect, provided you are still doing only trivial work inside your comprehension.