code: def add_source(self, source):
          
          if source is not None:
              self.sources.add(source)
              source.callback = self._receive
              if hasattr(source, 'start'):
                  source.start()
docs: Add a vehicle data source to the instance.
  
          The Vehicle instance will be set as the callback of the source, and the
          source will be started if it is startable. (i.e. it has a ``start()``
          method).