Zero cross detection with serial communication from Arduino to Raspberry Pi - raspberry-pi3

Question

I'm new at python and i'm setting up my school project that it's going to detect the zero cross for trigger a PWM pulse so i'm using Arduino and Raspberry Pi 3.
I'm trying to read from serial port of the Arduino for read it in the Raspberry Pi, after that what i need is build an if conditional with that data that i recieved.
Arduino sends from serial port ON and OFF message with this code
const int inputPin = 2;
int value = 0;
void setup() {
Serial.begin(9600);
pinMode(inputPin, INPUT_PULLUP);
}
void loop(){
value = digitalRead(inputPin); //digital read of pin
//sends a message to serial port depending of the value read
if (value == HIGH) {
Serial.println("ON");
}
else {
Serial.println("OFF");
}
delayMicroseconds(1000);
}
On Arduino serial read it shows continuously
OFF
ON
ON
ON
OFF
So on python i want to read those two values "ON" and "OFF" or at least one that when it takes a OFF value for example at the next 1 ms it turn on PWM pulse.
Something like:
if zerocross == OFF:
pi.hardware_PWM(heat,200,dutycycle)
This is my python code for read the data from serial port:
import time
try:
import serial
arduino = serial.Serial('/dev/ttyACM0', baudrate=9600, timeout=1.0)
arduino.setDTR(False)
time.sleep(1)
arduino.flushInput()
arduino.setDTR(True)
except (ImportError, serial.SerialException):
import io
class FakeArduino(io.RawIOBase):
def readline(self):
time.sleep(0.1)
return b'sensor = 0\toutput = 0\r\n'
arduino = FakeArduino()
with arduino:
while True:
try:
line = arduino.readline()
zerocross = line.decode('ascii', errors='replace')#, end=''
if zerocross == OFF:
print ('ok')
except KeyboardInterrupt:
print("Exiting")
break
this is the out in the python shell
Traceback (most recent call last):
File "/home/pi/Desktop/arduino python raspb.py", line 33, in <module>
if zerocross == OFF:
NameError: name 'OFF' is not defined
I've already tried to convert the serial data into a string but i get "built-in method decode" when i try to print the value. Please if someone can show me some other methods i would appreciate it so much.