pySerial: Connection with Arduino

Recently, I’ve trying to make sort of "light control" on Arduino. I use Raspberry Pi to send the control message via serial port (USB cable).

Here is the Arduino code:

void setup()
{
 Serial.begin(9600);
 pinMode(whiteled,OUTPUT);
}

void loop()
{
 if(Serial.available()>0)
 {
  char cmd = Serial.read();
  if(cmd == 'a')
  {
   
   while(1)
   {
    Serial.print("data received = ");
    Serial.print(cmd);
    digitalWrite(whiteled,HIGH);
    delay(1000);
    digitalWrite(whiteled,LOW);
   }
  }
 }
}

updating......