Creating ArcPy scripting tool with dropdown box for sql expression choice

Question

I am creating an ArcPy scripting tool to allow users to select a feature from a feature layer and then zoom to that selection. I have successfully created this tool and now am working on creating the parameters.

I want the user to be able to view a dropdown list of the allowable selections (taken from a field within the attribute table of the feature layer). I currently have the parameter set as a sql expression- however this would require the user to type in more information than they probably know. How can I create a populated dropdown list for the parameter that will correctly input the sql expression? I have attached my code below.

import arcpy
Selection = arcpy.GetParameterAsText(0)
mxd = arcpy.mapping.MapDocument("CURRENT")
dataFrame = arcpy.mapping.ListDataFrames(mxd, "*")[0] 
arcpy.SelectLayerByAttribute_management("PLSS.lyr","NEW_SELECTION", Selection)
dataFrame.zoomToSelectedFeatures()

Answer 1

From line 2, it looks like your code is intended as a Toolbox script tool. Under Toolbox -> Tool -> Properties -> Parameters tab, select the Selection parameter and click next to Filter in the Parameter Properties box. This will give you a drop-down - click "Value List." If you have expressions in mind for specific feature layers - parcels or addresses, for example - you can then input those as entries in the value list.

If you want to make it even more user friendly, you can populate the list with memorable names like "Select Parcels By Address" and have those be keys to a Python dictionary - "Select Parcels By Address" would return a SQL expression string to use in the query function.