old_contents: 
  import serial
  import sonic
  
  
  sonic_pi = sonic.SonicPi()
  connection = serial.Serial('/dev/tty.usbmodem1421', 115200)
  
  while True:
      line = connection.readline()
      command, argument = line.strip().split(' ', 1)
      if command == 'pad':
          number = int(argument)
          sonic_pi.run('cue :pad, number: {}'.format(number))
  
  
new_contents: 
  import serial
  import sonic
  import glob
  
  
  sonic_pi = sonic.SonicPi()
  connection = serial.Serial(glob.glob('/dev/tty.usbmodem*')[0], 115200)
  
  while True:
      line = connection.readline()
      command, argument = line.strip().split(' ', 1)
      if command == 'pad':
          number = int(argument)
          sonic_pi.run('cue :pad, number: {}'.format(number))
  
  
subject: Use `glob` to find an appropriate serial ttry