[SOLVED] python: compare old and new in a loop

ezekieldas · 03-23-2012, 04:07 PM

Without much formal programming, a bit of gleaning, trial, and error I've managed to mash together a daemon that is quite useful to me (BTW, just yesterday this involved a cron job and 2 shell scripts :-)). Anyhow, this has two functional problems for me:

1. How can I avoid 'the addrs are different' on first run without using a counter?
2. My logic is off but how can I store value of former loop in myipaddr_then in order
to do a compare with myipaddr_now ?

For initial testing, I can change b.txt at any time. The host, my.example.com is my server.

Code:

import time, os, sys
from daemon import runner

class App:
    def __init__(self):
        self.stdin_path = '/dev/null'
        self.stdout_path = '/dev/tty'
        self.stderr_path = '/dev/tty'
        self.pidfile_path =  '/tmp/foo.pid'
        self.pidfile_timeout = 5
    def run(self):

        myipaddr_then = 0

        while True:

         # myipaddr_now = os.popen('curl -s http://icanhazip.com').read()
         myipaddr_now = os.popen('curl -s http://my.example.com/b.txt').read()
         myipaddr_now = myipaddr_now.rstrip()

         if myipaddr_now == myipaddr_then:
          # print "ok"
          return
         else:
          print "the addrs are different"

         time.sleep(10)

         myipaddr_then = myipaddr_now

app = App()
daemon_runner = runner.DaemonRunner(app)
daemon_runner.do_action()

Dark_Helmet · 03-23-2012, 04:53 PM

Quote:

Originally Posted by ezekieldas

1. How can I avoid 'the addrs are different' on first run without using a counter?

You need to use a special value (or "flag") to indicate an initial state. You cannot avoid it. Whether it be a counter or something else. For your particular code, a good should-never-be-used-except-for-initialization Python value would be None. For instance, in your class initialization:

Code:

class App:
    def __init__(self):
        self.stdin_path = '/dev/null'
        self.stdout_path = '/dev/tty'
        self.stderr_path = '/dev/tty'
        self.pidfile_path =  '/tmp/foo.pid'
        self.pidfile_timeout = 5
        self.myipaddr_then = None

Then you utilize it in your "run" method like so:

Code:

    def run(self):

        while True:

         # myipaddr_now = os.popen('curl -s http://icanhazip.com').read()
         myipaddr_now = os.popen('curl -s http://my.example.com/b.txt').read()
         myipaddr_now = myipaddr_now.rstrip()

         if self.myipaddr_then != None and myipaddr_now != myipaddr_then:
          print "the addrs are different"
         else:
          # print "ok"
          pass

         time.sleep(10)

         self.myipaddr_then = myipaddr_now

The above code should also address your second question. It eliminated the overwrite-with-0 at the beginning of the loop.

EDIT:
There was a slight problem with my original post. I had to rearrange the conditional. I changed "return" to "pass" assuming you meant to continue the loop regardless if the addresses are the same. If I recall, pass is an empty statement, which would allow you avoid a syntax error for an empty else clause.

ezekieldas · 03-23-2012, 09:43 PM

Dark_Helmet -- thanks for your help. This got me to completion of the project.