Furnace Monitor

#!/usr/bin/python
import sys
import RPi.GPIO as GPIO
import time
import MySQLdb
import pickle
from datetime import datetime, date

pins = [17,18,22,23]
prev = [0,0,0,0]
on = [0,1,1,0]
try:
    f = open('state', 'r')
    prev = pickle.load(f)
except:
    print('no previous state')
db = MySQLdb.connect('localhost','monitor','***','furnace')

def checkInput(pin, prev_value, on_value):
    input = GPIO.input(pin)
    if (prev_value != input):
        print('%s changed: %s' %(pin,input))
        c = db.cursor()
        try:
            if (input == on_value):
                c.execute("""insert into log_event (pin) values (%d)""" % pin)
            else:
                c.execute("""update log_event set off_time = CURRENT_TIMESTAMP where pin = %d and off_time is null""" % pin)
            db.commit()
        except MySQLdb.Error, e:
            print "Unexpected error: %s" % e
            db.rollback()
        f = open('state','w')
        pickle.dump(prev, f)
        f.close()
    return input

def heartbeat():
    cur_input = [GPIO.input(pins[0]),GPIO.input(pins[1]),GPIO.input(pins[2]),GPIO.input(pins[3])]
    print('heartbeat: %s,%s,%s,%s' % tuple(cur_input))
    c = db.cursor()
    try:
        c.execute("""insert into heartbeat (p17,p18,p22,p23) values (%d,%d,%d,%d)""" % tuple(cur_input))
        db.commit()
    except:
        db.rollback()
    return datetime.now()

try:
    GPIO.setmode(GPIO.BCM)
    for i in range(len(pins)):
        GPIO.setup(pins[i],GPIO.IN)
        prev[i] = GPIO.input(pins[i])
    last_beat = heartbeat()
    while True:
        for i in range(len(pins)):
            prev[i] = checkInput(pins[i], prev[i], on[i])
        if ((datetime.now() - last_beat).total_seconds() >= 60):
            last_beat = heartbeat()
            f = open('state','w')
            pickle.dump(prev, f)
            f.close()
        time.sleep(0.5)
except:
    l = open('error.log','w')
    l.write(sys.exc_info()[0])
    l.close()