Python "While" Loop logic wrong?

Question

I have a Python script that queries a MySQL database every 5 seconds, gathering the latest three ID's for helpdesk tickets. I am using MySQLdb as my driver. But the issue is in my "while" loop, when I check if two arrays are equal. If they are NOT equal, I print "A new ticket has arrived." But this never prints! See my code:

import MySQLdb
import time

# Connect
db = MySQLdb.connect(host="MySQL.example.com", user="example", passwd="example", db="helpdesk_db", port=4040)
cursor = db.cursor()

IDarray = ([0,0,0])
IDarray_prev = ([0,0,0])

cursor.execute("SELECT id FROM Tickets ORDER BY id DESC limit 3;")
numrows = int(cursor.rowcount)
for x in range(0,numrows):
   row = cursor.fetchone()
   for num in row:
      IDarray_prev[x] = int(num)
cursor.close()
db.commit()

while 1:
   cursor = db.cursor()
   cursor.execute("SELECT id FROM Tickets ORDER BY id DESC limit 3;")

   numrows = int(cursor.rowcount)
   for x in range(0,numrows):
      row = cursor.fetchone()
      for num in row:
         IDarray[x] = int(num)

   print IDarray_prev, " --> ", IDarray
   if(IDarray != IDarray_prev): 
      print "A new ticket has arrived."

   time.sleep(5)
   IDarray_prev = IDarray
   cursor.close()
   db.commit()

Now when this ran, then I created a new ticket, the output looks like this:

[11474, 11473, 11472]  -->  [11474, 11473, 11472]
[11474, 11473, 11472]  -->  [11474, 11473, 11472]
[11474, 11473, 11472]  -->  [11474, 11473, 11472]
[11474, 11473, 11472]  -->  [11474, 11473, 11472]
[11475, 11474, 11473]  -->  [11475, 11474, 11473]
[11475, 11474, 11473]  -->  [11475, 11474, 11473]
[11475, 11474, 11473]  -->  [11475, 11474, 11473]
[11475, 11474, 11473]  -->  [11475, 11474, 11473]
[11475, 11474, 11473]  -->  [11475, 11474, 11473]

Where the format of my output is:

[Previous_Last_Ticket, Prev_2nd_to_last, Prev_3rd] --> [Current_Last, 2nd-to-last, 3rd]

Notice the change in numbers and, more importantly, the lack of "A new ticket has arrived"!

Answer 1

The problem is the following line:

IDarray_prev = IDarray

In Python, this makes IDarray_prev refer to the same underlying list as IDarray. Changes in one will be reflected in the other, because they both point to the same thing.

To make a copy of the list that you can use to compare later, try:

IDarray_prev = IDarray[:]

The [:] is Python slice notation that means "a copy of the whole list".

Answer 2

Python works with references, so you basically change both lists after the first while iteration (since they both have the same reference after you assign IDarray to IDarray_prev).

Try assigning a copy of IDarray using IDArray_prev = list(IDarray).