old_contents: #!/usr/bin/env python3
  import sys
  import serial
  import datetime
  
  if __name__=='__main__':
  	try:
  		ser = serial.Serial('COM1', 115200, timeout=1)
  		print("opened " + ser.portstr)
  	except serial.SerialException:
  		print('could not open port')
  		sys.exit(1)
  		pass
  	f = open(datetime.datetime.now().strftime("%y-%m-%d_%H-%M-%S") + '.csv', 'w+')
  	f.write('10ms;x;y;z;delta\n')
  	i = 0
  	line = ser.readline().decode('utf-8') # Truncate first read line
  	try:
  		while True:
  			line = ser.readline().decode('utf-8')
  			if line:
  				f.write(str(i) + ';' + line)
  				i = i + 1
  			print(line)
  	except KeyboardInterrupt:
  		pass
  
  	f.close()
  	ser.close()
  
new_contents: #!/usr/bin/env python3
  import sys
  import serial
  import datetime
  
  if __name__=='__main__':
  	try:
  		ser = serial.Serial('COM1', 115200, timeout=1)
  		print("opened " + ser.portstr)
  	except serial.SerialException:
  		print('could not open port')
  		sys.exit(1)
  		pass
  	f = open(datetime.datetime.now().strftime("%y-%m-%d_%H-%M-%S") + '.txt', 'w+')
  	f.write('10ms\tx\ty\tz\tdelta\n')
  	i = 0
  	line = ser.readline().decode('utf-8') # Truncate first read line
  	try:
  		while True:
  			line = ser.readline().decode('utf-8')
  			if line:
  				f.write(str(i) + '\t' + line.replace(';', '\t'))
  				i = i + 1
  			print(line)
  	except KeyboardInterrupt:
  		pass
  
  	f.close()
  	ser.close()
  
subject: Use TAB instead of ;