Subprocess 使用python子流程使用ssh登录到远程网络设备

在上面的代码中，它成功登录到网络设备，但我无法通过输入exit退出显示>的设备程序。我不知道我错过了什么，请帮忙

def ssh(ip):
   try:
      cmd = "plink -ssh {}@{} -pw {} -q + " exit" ".format( u , ip , pw )
      process = subprocess.Popen( cmd , shell=True , stdin=subprocess.PIPE , 
      stdout=subprocess.PIPE,stderr=subprocess.PIPE, encoding='utf-8')
      a,b = process.communicate()
      print( "a:{},b:{}".format( a , b ) )
      succ = ''.join( str( e ) for e in a )
      err = ''.join(str(e) for e in b)
      succ = succ.rstrip()
      process.stdout.close()
      print("A-1:{}".format(err))
      succout=process.stdout.readlines()
      print("Success:{}".format(succ))
                
      if "Access granted:" in succ:
         process.communicate(b'\r\n')
         process.stdin.write( b"exit\r\n" )               
         print( "Successfully authenticated" )
         output = 'Success' + " " + u
         ssh_login_status = output
         final_list[ 'SSH_AUTH' ].append( ssh_login_status )
         final_list[ 'IP' ].append( ip )
         break
                
    else:
         output = "Unable to establish SSH connection"
         print( "Invalid authentication" )
         final_list[ 'SSH_AUTH' ].append( output )  # changed later to error message suggested
         final_list[ 'IP' ].append( ip )


   except Exception as e:
                 raise e
            
             except socket.error as e:
                 output = "Connection Error"
                 ssh_login_status = output
                 final_list[ 'SSH_AUTH' ].append( output )
                 print( output )
             process.kill()