code: def field_from_json(self, key_and_type, json_value):
          
          assert ':' in key_and_type
          key, type_code = key_and_type.split(':', 1)
          from_json = self.field_function(type_code, 'from_json')
          value = from_json(json_value)
          return key, value
docs: Convert a JSON-serializable representation back to a field.