code: def unmarshal(self, v):
          
          if self.units:
              # Note that we don't want to cast to type in this case!
              if not isinstance(v, Quantity):
                  v = self.units(v)
          elif not isinstance(v, self.type):
              v = self.type(v)
          return v
docs: Convert the value from parsed JSON structure to native python representation.
  
          By default this will leave the value as-is since the JSON parsing routines
          typically convert to native types. The exception may be date strings or other
          more complex types, where subclasses will override this behavior.