Now that you have created the VMD file that generates the ACK and NACK messages, the next step is to create the Python script that determines whether to reject an incoming message. When a message is rejected, a NACK message will be returned to the application that sent the incoming message.

For example, suppose that you want your channel to reject all incoming messages that have a value of A04 in the first field of the EVN segment (also known as EVN.1). The following Python function enables you to perform this task:

def reject_message(message):
   all_segments = message.split('\r')
   for segment in all_segments:
      if segment.startswith('EVN'):
         evn_fields = segment.split('|')
         if evn_fields[1] == 'A04':
            return True
         else:
            return False  # Not an A04 event.
   return False  # No EVN segment.

This function returns True if EVN.1 contains A04. It returns False if EVN.1 does not contain A04 or if no EVN segment exists in this message.

For best results, the reject_message() function shown above is saved as a separate Python module. This enables you to reuse this script if you need to create a VMD file that filters the incoming messages so that messages that have been rejected are not processed. (See Creating the Filtering VMD File for an example.)

To save this function as a Python module:

• Save this code in a file on your system named reject_evn.py. This creates a module named reject_evn, as the name of a Python module is always the same as the name of the file in which the module is found.

• Ensure that Chameleon can find this file when it is searching for Python modules (see Searching for Modules for details on how to do this).

• Import the reject_evn module in any Python scripts that use this function. An example of this is shown in the next section.