setting output param 'symbology' fails to display my tool's output

Discussion created by crispincooper on Mar 21, 2011
Latest reply on Apr 6, 2011 by crispincooper

My geoprocessing tool takes a feature class as input, calculates an extra field and adds it to the same feature class as output (a derived output parameter).  The tool's parameters dialog allows me to set symbology, which is nice because I can control how the results are displayed.

Starting with a blank map, the tool does its calculation and then loads a layer with the desired symbology - so far so good.  However if there is already a layer in the map that comes from the same feature set (showing for example some different data from the same table, or 'single symbol' display), then nothing happens on tool completion.  The desired behaviour would be to add an extra layer with the desired symbology to show results.

Is there a work-around to make this happen?  I see arcpy.mapping allows me to add layers to the document but I'd prefer to avoid that as I need compatibility with v9.3.

Many thanks, C.