RPI to Arduino slave via I2C - Can't print to serial monitor

I have a simple raspberry Pi program that receives user input, either 1 or 0, that turns on or off pin13 on Arduino (taken from here). However, I can't print the messages in the serial monitor. I've tried in the receiveEvent() function and loop(). Why is this?

Raspberry Pi:

from smbus import SMBus

addr = 0x8
bus = SMBus(1) #indicates /dev/i2c-1

num = 1

print("enter 1 for on or 0 for off")
while num == 1:
	state = input(">>>>   ")
	
	if state == "1":
		bus.write_byte(addr, 0x1)
	elif state == "0":
		bus.write_byte(addr, 0x0)
	else:
		num = 0

Arduino:

#include <Wire.h>
char new_data;
char old_data;
void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(13, OUTPUT);
  Wire.begin(0x8);
  Wire.onReceive(receiveEvent);
}

void receiveEvent(int howMany) {
  while(Wire.available()) {
    old_data = new_data;
    new_data = Wire.read();
    digitalWrite(13,new_data);
    Serial.print(new_data);
  }
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(100);
  if(old_data!=new_data){
    Serial.print(new_data);
  }
}

Try to print the number as a hexadecimal.
Serial.print(new_data, HEX);