code: def _generate_union_class_repr(self, data_type):
          
          self.emit('def __repr__(self):')
          with self.indent():
              self.emit("return '{}(%r, %r)' % (self._tag, self._value)".format(
                  class_name_for_data_type(data_type),
              ))
          self.emit()
docs: The __repr__() function will return a string of the class name, and
          the selected tag.