code: def _generate_union_class_vars(self, data_type):
          
          lineno = self.lineno
          if data_type.catch_all_field:
              self.emit("_catch_all = '%s'" % data_type.catch_all_field.name)
          elif not data_type.parent_type:
              self.emit('_catch_all = None')
  
          # Generate stubs for class variables so that IDEs like PyCharms have an
          # easier time detecting their existence.
          for field in data_type.fields:
              if is_void_type(field.data_type):
                  field_name = fmt_var(field.name)
                  self.emit('# Attribute is overwritten below the class definition')
                  self.emit('{} = None'.format(field_name))
  
          if lineno != self.lineno:
              self.emit()
docs: Adds a _catch_all_ attribute to each class. Also, adds a placeholder
          attribute for the construction of union members of void type.