Simple code to read values from an analogue line sensor (using an ADC as this is for the Pi) and say if a line is detected or not. View https://theraspberryblonde.wordpress.com/2017/01/08/qre1113-analogue-line-sensor-with-raspberry-pi/ to see more details about how to use the code.

lineSensor.py

#!/usr/bin/python
 
import spidev
import time

# which ADC input the sensor is connected to
sensor = 0
# the boundary between no line detected and a line detected
boundary = 30

# open the spi connection for the ADC
spi = spidev.SpiDev()
spi.open(0, 0)

# read the ADC value for the given input
def getADC(adcnum):
    r = spi.xfer2([1, 8 + adcnum << 4, 0])
    data = ((r[1] & 3) << 8) + r[2]
    return data

# find the average value where there isn't a line
# NOTE: for calibration to work, the robot must
# not be over the line at the start of the program
def calibrate():
    val = 0
    for i in range(100):
        val += getADC(sensor)
        time.sleep(0.05)
    val /= 100
    return val

noLineVal = calibrate()

while True:
    # read the line sensor value
    lineVal = getADC(sensor)
    
    # if value higher than value for no line
    if lineVal > noLineVal + boundary:
        print "centre on line"
    else:
        print "not on line"
    time.sleep(0.5)