code: def _resolve_value(self, value, environment=None):
          
  
          # here we care about Undefined values
          if value == values.Undefined:
              if isinstance(self._missing, type) and issubclass(self._missing, Missing):
                  # instantiate the missing thing
                  missing = self._missing(self, environment)
  
                  # invoke missing callback
                  # the default is to raise a MissingValue() exception
                  value = missing(value)
  
              elif hasattr(self._missing, "__call__"):
                  value = self._missing()
  
              else:
                  # we just assign any value
                  value = self._missing
  
          return value
docs: Resolve the value.
  
          Either apply missing or leave the value as is.
  
          :param value: the value either from deserialize or serialize
          :param environment: an optional environment