[lmm]

Python for comprehensions are middle-ended for some reason: part of it goes backwards and part of it goes forwards. Read it as:

    planets_flat = [planet for planet in episode['planets'] for episode in episodes.values()]

and it makes a lot more sense. In most languages with list comprehensions you'd write the thing you were iterating over first, e.g. (Scala):

    val planetsFlat = for { episode <- episodes.values; planet <- episode(planets) } yield planet