code: def dict_value_hint(key, mapper=None):
      
      if mapper is None:
          mapper = identity
  
      def hinter(data):
          return mapper(data.get(key))
  
      return hinter
docs: Returns a function that takes a dictionary and returns value of
      particular key. The returned value can be optionally processed by `mapper`
      function.
  
      To be used as a type hint in :class:`OneOf`.