What I learned

  I caught a glimpse of a tome on Python wizardry for the big
  leagues. Didn't exactly float my boat, but a couple nuggets
  stuck like gum on a sidewalk:

  Picking your container flavor can be a sneaky way of documentation,
  leaving breadcrumbs for the next poor soul, hinting whether you're
  cool with duplicate values or not. (But other times it's just about
  efficiency.)

  These days, you can slap "Final" on something (at least for
  module variables) to tell it's not to be changed!

from typing import Final

a : Final = 2