import RPi.GPIO as IO          #calling header file which helps us use GPIO’s of PI
import time                            #calling time to provide delays in program
IO.setwarnings(False)           #do not show any warnings
IO.setmode (IO.BCM)         #we are programming the GPIO by BCM pin numbers. (PIN35 as ‘GPIO19’)
IO.setup(19,IO.OUT)           # initialize GPIO19 as an output.
p = IO.PWM(19,100)          #GPIO19 as PWM output, with 100Hz frequency
p.start(0)                              #generate PWM signal with 0% duty cycle
while 1:                               #execute loop forever
    for x in range (50):                          #execute loop for 50 times, x being incremented from 0 to 49.
        p.ChangeDutyCycle(x)               #change duty cycle for varying the brightness of LED.
        time.sleep(0.1)                           #sleep for 100m second

    for x in range (50):                         #execute loop for 50 times, x being incremented from 0 to 49.
        p.ChangeDutyCycle(50-x)        #change duty cycle for changing the brightness of LED.
        time.sleep(0.1)                          #sleep for 100m second