I want to compare each polygon from one shapefile with all the polygons from another shapefile.
When one polygon is geometrically identical to a polygon on the other shapefile, I want to copy the value of field "code" of the second shapefile to the field "code" of the first shapefile.
I wrote the following code:

fc1 = ur'D:\polygons1.shp'
fc2 = ur'D:\polygons2.shp'
fields = ['SHAPE@','code']
with arcpy.da.UpdateCursor(fc1,fields) as cur1:
    with arcpy.da.SearchCursor(fc2,fields) as cur2:
        for row1 in cur1:
            for row2 in cur2:
                row1[1] = row2[1] if(row1[0]==row2[0]) else ''

When I run the above code, the values from "code" are not copied plus there are no errors.
Any ideas?

  • 1
    The object equivalence test will always fail. You need to use a geometry comparison operator. If you do a spatial search of fc2, your program won't be O(N^2). – Vince Oct 19 '18 at 10:34
  • Have you considered using a spatial join? – Andy Oct 19 '18 at 19:57
  • @Andy I did use. I just wondering why the above code does not work. – Panos Oct 20 '18 at 19:41

As commented you cant compare geometries (SHAPE@) using equal to operator (==), you need a geometry comparison. Use the equals method:

Indicates if the base and comparison geometries are of the same shape type and define the same set of points in the plane.

import arcpy
arcpy.env.workspace = r'C:\Default.gdb' #Change
fc1 = 'polygons1' #Change
fc2 = 'Polygons2' #Change
fields = ['SHAPE@','code'] #Change

fc2_records = [i for i in arcpy.da.SearchCursor(fc2,fields)]

with arcpy.da.UpdateCursor(fc1,fields) as cursor:
    for row in cursor:
        for record in fc2_records:
            if row[0].equals(record[0]):

(If you have large datasets there will be faster ways. You could try merging the feature classes, sort by id and delete duplicates on shape field. Or do a select by location to find overlapping features and then pass these to the cursors.)

| improve this answer | |
  • Probably ought to test for objectid1 != objectid2, but that means adding id columns to the fields list – Vince Oct 19 '18 at 12:10
  • Equality is not what I want to compare, Identicality is the one I want. @Vince OIDs between the two shapefiles are completely irrelevant. – Panos Oct 20 '18 at 19:46
  • Equals method will return true for identical geometries – BERA Oct 20 '18 at 19:50
  • Doh. The OID2>OID1 test is for two queries on a single source. – Vince Oct 20 '18 at 22:21

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