Helper for _AddMessageMethods().

def _AddSerializePartialToStringMethod(message_descriptor, cls):
  """Helper for _AddMessageMethods()."""

  def SerializePartialToString(self):
    out = BytesIO()
    self._InternalSerialize(out.write)
    return out.getvalue()
  cls.SerializePartialToString = SerializePartialToString

  def InternalSerialize(self, write_bytes):
    for field_descriptor, field_value in self.ListFields():
      field_descriptor._encoder(write_bytes, field_value)
    for tag_bytes, value_bytes in self._unknown_fields:
      write_bytes(tag_bytes)
      write_bytes(value_bytes)
  cls._InternalSerialize = InternalSerialize