code: def check_pid_file(self):
          
          if not os.path.exists(self.pid_file):
              return 0
  
          self.show_status()
          raise self.Error("""\n\
              pid_file
              %s
              already exists. There are two possibilities:
  
                 1) There's an another instance of PyFlowScheduler running
                 2) The previous scheduler didn't exit in a clean way
  
              To solve case 1:
                 Kill the previous scheduler (use 'kill pid' where pid is the number reported in the file)
                 Then you can restart the new scheduler.
  
              To solve case 2:
                 Remove the pid_file and restart the scheduler.
  
              Exiting""" % self.pid_file)
docs: This function checks if we are already running the :class:`Flow` with a :class:`PyFlowScheduler`.
          Raises: Flow.Error if the pid file of the scheduler exists.