code: def lists_from_env(key, default=""):
      
      try:
          val = os.environ.get(key, default)
          lists = val.split(":")
          return [i.split(',') for i in lists]
      except (KeyError, ValueError):
          return []
docs: Splits a string in the format "a,b:c,d,e:f" into
      [('a', 'b'), ('c', 'd', 'e'), ('f', )]