Serial Communication works only when serial monitor is active

Okay heres some of my code, i thought it is not needed, because it is actually very simple:

Arduino Code:
in loop:

int sensorValueL = analogRead(A5);
int sensorValueR = analogRead(A6);
Serial.print(sensorValueL); //Write the value of the photoresistor to the serial monitor.
Serial.print(","); //Write the value of the photoresistor to the serial monitor.
Serial.print(sensorValueR); //Write the value of the photoresistor to the serial monitor.
Serial.print("\n");

and on the python side i try to read it this way:

#Only once
import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)

#in loop:
values = ser.readline()

Do some processing here.