code: def render(self):
          
          size_format = self.size_primitive.fmt
  
          if self.value is None:
              return size_format, [-1]
  
          value = self.render_value(self.value)
  
          size = len(value)
  
          fmt = "%s%ds" % (size_format, size)
  
          return fmt, [size, value]
docs: Returns the ``struct`` format and list of the size and value.
  
          The format is derived from the size primitive and the length of the
          resulting encoded value (e.g. the format for a string of 'foo' ends
          up as 'h3s'.
  
          .. note ::
            The value is expected to be string-able (wrapped in ``str()``) and is
            then encoded as UTF-8.