code: def single_shot(self):
          
          num_launched = 0
  
          # Get the tasks that can be executed in each workflow.
          tasks = []
          for work in self.flow:
              try:
                  task = work.fetch_task_to_run()
  
                  if task is not None:
                      tasks.append(task)
                  else:
                      # No task found, this usually happens when we have dependencies.
                      # Beware of possible deadlocks here!
                      logger.debug("No task to run! Possible deadlock")
  
              except StopIteration:
                  logger.info("All tasks completed.")
  
          # Submit the tasks and update the database.
          if tasks:
              tasks[0].start()
              num_launched += 1
  
              self.flow.pickle_dump()
  
          return num_launched
docs: Run the first :class:`Task` than is ready for execution.
  
          Returns:
              Number of jobs launched.