code: def walk(self, o):
          
          if isinstance(o, dict):
              d = o if self._dx is None else self._dx(o)
              return {k: self.walk(v) for k, v in d.items()}
          elif isinstance(o, list):
              return [self.walk(v) for v in o]
          else:
              return o if self._vx is None else self._vx(o)
docs: Walk a dict & transform.