One tricky problem with using global variables in Python scripts in Chameleon is how to initialize them.

For example, suppose that you want to define a counter in Python to assign a global message ID to each message that you generate. The following code will not work, as the globalMessageID variable needs to be initialized before it can be incremented:

globalMessageID += 1

However, if you try to initialize globalMessageID simply by assigning to it, it will be reset every time the script is called:

globalMessageID = 1

The solution is to use an exception handler to check whether globalMessageID is defined:

try:
   globalMessageID   # test whether globalMessageID is defined yet
   print "Have globalMessageID - value is", globalMessageID
   globalMessageID += 1
   print "Incremented globalMessageID to", globalMessageID
except NameError:
   print "Getting a new globalMessageID"
   globalMessageID = 1
   print "Set globalMessageID to", globalMessageID

This code references the variable globalMessageID inside a try statement. If globalMessage is not yet defined, a NameError exception is detected, and Python jumps to the code inside the except NameError statement. This code sets globalMessageID to an initial value of 1.

	You can use this technique to initialize any global variable that you want to use in a Python script. For an example of how to use this global variable initialization technique to detect message inactivity in Iguana, see Detecting Inactivity When Multiple Message Types Are Sent Through a Single Channel.