aa.py

import serial #Serial imported for Serial communication
import time #Required to use delay functions

ArduinoSerial = serial.Serial('/dev/cu.usbmodem1421',9600) #Create Serial port object called arduinoSerialData
time.sleep(2) #wait for 2 secounds for the communication to get established

print (ArduinoSerial.readline()) #read the serial data and print it as line
print ("Enter 1 to turn ON LED and 0 to turn OFF LED")


while True: #Do this forever
    var = input() #get input from user
    print ("you entered", var) #print the intput for confirmation

    if var == '1': #if the value is 1
        ArduinoSerial.write(b'1') #send 1
        print ("LED turned ON")
        time.sleep(1)

    if var == '0': #if the value is 0
        ArduinoSerial.write(b'0') #send 0
        print ("LED turned OFF")
        time.sleep(1)

ard.cpp

void setup() { 
  Serial.begin(9600); //initialize serial COM at 9600 baudrate
  pinMode(2, OUTPUT); //make the LED pin (13) as output
  digitalWrite (2, LOW);
  
  Serial.println("Hi!, I am Arduino");
}
 
void loop() {
  while (Serial.available()){
    data = Serial.read();
  }

  if (data == '1')
    digitalWrite (2, HIGH);

  else if (data == '0')
    digitalWrite (2, LOW);
}