code: def n_executions(self):
          
          pipeline = self.tiger.connection.pipeline()
          pipeline.exists(self.tiger._key('task', self.id))
          pipeline.llen(self.tiger._key('task', self.id, 'executions'))
          exists, n_executions = pipeline.execute()
          if not exists:
              raise TaskNotFound('Task {} not found.'.format(
                  self.id
              ))
          return n_executions
docs: Queries and returns the number of past task executions.