Using Select By Location to update field in feature class using ArcPy?

Question

I am using Arc 10.1 and I am working on a script that takes a value from a field within one shape file and updates a field in another shape file based on its location. Basically I have smaller parcels that reside within larger areas and I want to assign the name of the larger area to a field within the smaller parcels.

The issue I am having is that when I run the script, every parcel ends up having the same "area". I believe its because its essentially writing each area to the parcel and then rewriting it again and again until there are no more areas to loop through. Can someone provide some advice?

#Make a feature layer for the neighourhoods
arcpy.MakeFeatureLayer_management(neighbourhood, "neighbourhoodLayer")
arcpy.MakeFeatureLayer_management(parcel, "parcelLayerTest1")
#Setup cursor to update new NBHD field
with arcpy.da.UpdateCursor(parcel, (parcelIDField,nbhdField)) as parRows:
    for par in parRows:
        # Create a query string for the current patrol zone 
        #parcelIDstring = par[0]
        #queryString = '"' + str(parcelIDField) + '" =' + "'" + str(parcelIDstring) + "'"

        #Select parcels based on location contained within neighbourhood
        arcpy.SelectLayerByLocation_management("parcelLayerTest1", "HAVE_THEIR_CENTER_IN", neighbourhood)

        nbhdTable = arcpy.SearchCursor(neighbourhood)
        for nbhd in nbhdTable:
            nbhdValue = nbhd.getValue("Neighbourh")
            if nbhdValue:
                par[1] = nbhdValue
                parRows.updateRow(par)

Answer 1

The problem with your code lies in the logical flow and that you are selecting all the parcel features each time your arcpy.SelectLayerByLocation_management is called. You should re-work your code flow to:

1. First cursor/select each neighborhood feature (within cursor use select by attribute)
2. Select by location using neighbohood selected features to select parcel features
3. Perform update cursor on selected parcel features or use Calculate Field tool to perform the value update grabbed from step 1 value

Answer 2

Try using spatial join instead. It will grab the name of the larger area for you and join it to the smaller areas. If you just need to do it once, I would just use the GUI tool, as setting up the field maps and join rules to make things tidy can be a bit cumbersome in Python for a one-time use. If you do want to automate it, you should read up on the FieldMap() and FieldMappings() objects in arcpy.mapping. These aren't strictly necessary, but it allows you to only include the fields that you want in your output feature class.