如何在满足条件而不是无限的情况下仅打印一次 else 或 if 语句而不中断

Question

In the below python code I am checking if google is up when it is up its printing "google.com is up" infinite amount of times, I want to print just once and wait until it'll go down and when google goes down I want to print "google.com is down" just once and again when google goes up print "google.com is up" like wise continue the loop without breaking.

hostname = "google.com" #example
while True :
  response = os.system("ping -c 1 " + hostname + "> /dev/null")

#and then check the response...
  if response == 0:
    print (hostname, 'is up!')
  else :
    print (hostname, 'is down!')

Answer 1

You can add variable for checking last value. Eg if response is 0, setting variable value and print. If response value doesn't change, not print and change variable value

Something like that:

hostname = "google.com" #example
state = -1
while True :
  response = os.system("ping -c 1 " + hostname + "> /dev/null")
  if response == 0 and state != 0:
      print(hostname, 'is up!')
      state = 0
  elif state != 1:
      print (hostname, 'is down!')
      state = 1

Answer 2

I'm not sure what is True, but if you are in a loop, and want to do something only the first time it's triggered, and not subsequent times, then you would need to flag the first time vs. subsequent times. Eg, before the while statement, initialize two variables, eg, first.true = 1 and first.false = 1. Inside the if statement, only print if first.true = 1, and after that (but still inside the if statement), set it to 0. This ensures that it will only print the first time it gets there. And likewise with first.false inside the else statement.

Answer 3

You can add a break statement to exit while loop, for example like that:

hostname = "google.com" #example
while True :
  response = os.system("ping -c 1 " + hostname + "> /dev/null")

#and then check the response...
  if response == 0:
    print (hostname, 'is up!')
    break
  else :
    print (hostname, 'is down!')
    break

However, i am not understanding your question so good, using a state variable like in the other answer should be ok too!!

Answer 4

You can create an update function with a static last_update string. The update function will not print the same output twice.

def update(st):
    if not hasattr(update, "last_update"):
        update.last_update = ""
    if update.last_update != st:
        print(st)
        update.last_update = st


while True:
    hostname = "www.google.com"
    response = os.system("ping " + hostname)

    # and then check the response...
    if response == 0:
        update(f"{hostname} 'is up!")
    else:
        update(f"{hostname} 'is down!")

Answer 5

In the below code its continuously pinging my home server printing 'down' only onetime when its down and waiting, When the server is up printing 'up' only onetime then again when the server goes down it'll print (down) only onetime.

hostname = "172.16.0.96" #example


while True :
    while True :
        response = os.system("ping -c 1 " + hostname + "> /dev/null")
        if response != 0:
            print("down")
            break
    while True :
        response = os.system("ping -c 1 " + hostname + "> /dev/null")
        if response == 0:
            print("up")
            break