arcpy - Select by Location in an Update Cursor

Question

I have parcels and a buffer around greenways. I'm trying to populate a field in the parcel fc with a '1' if the parcel intersects the buffer. I can get that fine with an update cursor and select layer by location. My problem is getting out of the cursor with all of the records rather than just those selected. Here is my code:

arcpy.management.AddField(regionParcels,"BufferParcels","SHORT")
arcpy.management.MakeFeatureLayer(regionParcels,"layerParcels")

with arcpy.da.UpdateCursor("layerParcels","BufferParcels") as cursor:
    arcpy.management.SelectLayerByLocation("layerParcels",'INTERSECT',bigBuffer)
    for row in cursor:
        row[0] = 1
        cursor.updateRow(row)

arcpy.management.SelectLayerByAttribute("layerParcels",'CLEAR_SELECTION')
arcpy.management.CopyFeatures("layerParcels",markedParcels)

With this code, every record is in the output, but they all have '1' in that field. If I drop the 'clear selection' command, I get just the selected features with the field populated, but I need the entire dataset. I've tried moving the clear command into the for loop, but that didn't work either. Any other suggestions?

Answer 1

I'm still not sure why it was doing what it was doing, but I got it to do what I want. In case others have the same issue, here is what worked.

arcpy.management.AddField(resParcels,"BufferParcels","SHORT")
arcpy.management.MakeFeatureLayer(resParcels,"layerParcels")

arcpy.management.SelectLayerByLocation("layerParcels",'INTERSECT',bigBuffer)

with arcpy.da.UpdateCursor("layerParcels","BufferParcels") as cursor:    
    for row in cursor:
        row[0] = 1
        cursor.updateRow(row)

arcpy.management.SelectLayerByLocation("layerParcels",selection_type = 'SWITCH_SELECTION')

with arcpy.da.UpdateCursor("layerParcels","BufferParcels") as cursor:
    for row in cursor:
        row[0] = 0
        cursor.updateRow(row)

arcpy.management.SelectLayerByAttribute("layerParcels",'CLEAR_SELECTION')
arcpy.management.CopyFeatures("layerParcels",markedParcels)

Answer 2

All you need to do is use a Cursor spatial_filter:

The issue with your code is that the select by attributes does nothing to a Cursor. Selecting a layer modifies the OID set of the layer object and the cursor only looks at the raw SQL table.

If you want to filter a Cursor, you need to use spatial filters and where clauses:

Here's some updated code from my unedited answer that solves exactly the problem you had. The making a layer and copying features can be run in a separate operation to the flagging, so I made it into a function.

import arcpy
import os
     
def flag_parcel(parcel_path: os.PathLike, buffer: arcpy.Polyline):
    desc = arcpy.Describe(parcel_path)
    if 'BufferParcels' in [f.name for f in desc.fields]:
        arcpy.management.DeleteField(parcel_path, 'BufferParcels')
    
    # Create the field and set it to 0 (default)
    arcpy.management.AddField(parcel_path, 'BufferParcels', 'SHORT')
    
    # Set the BufferParcels field to 1 for all parcels that intersect the buffer
    with arcpy.da.UpdateCursor(parcel_path, ['BufferParcels'], spatial_filter=buffer) as cursor:
        for row in cursor:
            row = dict(zip(cursor.fields, row))
            row['BufferParcels'] = 1
            cursor.updateRow(list(row.values()))
    
def main():
    resParcels = r"<path>"
    bigBuffer: arcpy.Polygon = arcpy.Polygon()
    flag_parcel(resParcels, bigBuffer)
    
    # If you have multiple buffers, you can use the following code to flag parcels for each buffer
    # bigBuffers: List[arcpy.Polygon] = [arcpy.Polygon(), arcpy.Polygon(), ...]
    # for buffer in bigBuffers:
    #     flag_parcel(resParcels, buffer)
    # 
    # Or Merge the buffers into a single buffer
    # bigBuffer: arcpy.Polygon = [bigBuffers[0].union(buf) for buf in bigBuffers[1:]][0]
    # flag_parcel(resParcels, bigBuffer)

if __name__ == "__main__":
    main(