code: def fill_into_dict(self, items, dest):
          
          for item in items:
              dest[self.key(item)].append(item)
docs: Take an iterable of `items` and group it into the given `dest` dict,
          using the :attr:`key` function.
  
          The `dest` dict must either already contain the keys which are
          generated by the :attr:`key` function for the items in `items`, or must
          default them suitably. The values of the affected keys must be
          sequences or objects with an :meth:`append` method which does what you
          want it to do.