python explain

#!/usr/bin/env python

# servo_demo.py
# 2016-10-07
# Public Domain

# servo_demo.py          # Send servo pulses to GPIO 4.
# servo_demo.py 23 24 25 # Send servo pulses to GPIO 23, 24, 25.

import sys
import time
import random
import pigpio

NUM_GPIO=32

MIN_WIDTH=1000
MAX_WIDTH=2000

step = [0]*NUM_GPIO
width = [0]*NUM_GPIO
used = [False]*NUM_GPIO

pi = pigpio.pi()

if not pi.connected:
   exit()

if len(sys.argv) == 1:
   G = [4]
else:
   G = []
   for a in sys.argv[1:]:
      G.append(int(a))
  
for g in G:
   used[g] = True
   step[g] = random.randrange(5, 25)
   if step[g] % 2 == 0:
      step[g] = -step[g]
   width[g] = random.randrange(MIN_WIDTH, MAX_WIDTH+1)

print("Sending servos pulses to GPIO {}, control C to stop.".
   format(' '.join(str(g) for g in G)))

while True:

   try:

      for g in G:

         pi.set_servo_pulsewidth(g, width[g])

         # print(g, width[g])

         width[g] += step[g]

         if width[g]<MIN_WIDTH or width[g]>MAX_WIDTH:
            step[g] = -step[g]
            width[g] += step[g]

      time.sleep(0.1)

   except KeyboardInterrupt:
      break

print("\nTidying up")

for g in G:
   pi.set_servo_pulsewidth(g, 0)

pi.stop()