Not sure if this will help, but here is a function I found (I think in these forums somewhere, apologies to the original contributor). Isn't as nice as being able to step through, but (usually) gives the offending line's number in the results dialog.### Geoprocessing script traceback error info ###
import os, sys, traceback
def trace():
tb = sys.exc_info()[2]
tbinfo = traceback.format_tb(tb)[0] # script name + line number
line = tbinfo.split(", ")[1]
# Construct the pathname to this script. Get the pathname
# to the folder containing the script using sys.path[0].
# Modify line below and replace 'myscript.py' with the name of
# this script.
filename = sys.path[0] + os.sep + "ScriptFileName.py" #<--- Insert filename
# Get Python syntax error
synerror = traceback.format_exc().splitlines()[-1]
return line, filename, synerror
## Try-Except clause format:
# try:
# your code here
# except arcpy.ExecuteError:
# for msg in range(0, arcpy.GetMessageCount()):
# if arcpy.GetSeverity(msg) == 2:
# arcpy.AddReturnMessage(msg)
# except:
# line, filename, err = trace()
# arcpy.AddError("Python error on " + line + " of " + filename)
# arcpy.AddError(err)
# print "Python error on " + line + " of " + filename
# print err
