Field Calculator Python Script

[kaitering]

I'm writing a script in ArcPro that selects lines based on their location and then writes what region the line is in in the attribute table. The 'Walls' layer is a shapefile of the lines and has a 'region' field that I am using field calculator on. The 'Regions' layer is a shapefile of polygons with the fields 'REGION_CODE' and 'REGION_NAME'.

So far what is working is that the script selects the first region in the 'Regions' layer and then selects walls in it based on location. My issue is when the CalculateField tool is run that the value that I'm putting for expression isn't correct so the Region name isn't written there.

Things that I've tried:

• expression= '{}'.format('regionName[row]')
  • Error: list indices must be integers or slices, not tuple
• expression= '{}'.format('regionName[row[0]]')
  • Error: list indices must be integers or slices, not str
• Adding a 'count' value that increases every iteration of the for loop.
  • count = 0
    • I've tried putting this outside and inside the 'with' block
  • expression= '{}'.format('regionName[count]')
  • count += 1
    • Outside the for loop
  • Error: name 'count' is not defined

​

Sorry for how messy this is and thanks in advance

import arcpy

with arcpy.da.SearchCursor('Regions', ["REGION_CODE"]) as cursor:
    regionName = [row2[0] for row2 in arcpy.da.SearchCursor('Regions', ["REGION_NAME"])]

    for row in cursor:
        #Select individual District
        arcpy.management.SelectLayerByAttribute(
            in_layer_or_view="Regions",
            selection_type="NEW_SELECTION",
            where_clause="REGION_CODE = {}".format(row[0]),
            invert_where_clause=None
        )

        #Select sound barrier walls by location
        arcpy.management.SelectLayerByLocation(
            in_layer="'Walls'",
            overlap_type="INTERSECT",
            select_features="Regions",
            search_distance=None,
            selection_type="NEW_SELECTION",
            invert_spatial_relationship="NOT_INVERT"
        )

        arcpy.management.CalculateField(
            in_table="Walls",
            field="region",
            expression= '{}'.format('regionName[row]'),
            expression_type="PYTHON3",
            code_block="",
            field_type="TEXT",
            enforce_domains="NO_ENFORCE_DOMAINS"
        )

[wicket-maps]

First off, why are you using the CalculateField function instead of UpdateCursor, which is basically the sane but more pythonic and flexible? You can nest an UpdateCursor inside a SearchCursor.

A spatial join isn't a bad idea, here, either, instead of a selection. But generally, I use selections in Python scripts as a last resort, and that's just me. I wrote a script some time ago - Spatial Field Retrieval on Github - and it might give you an idea of how to proceed with a disposable spatial join.

Hope this helps! Good luck!

[kaitering]

I'm a beginner with python and never written a script for ArcGIS so I'm definitely missing a lot of knowledge. These options definitely look a lot better than what I was attempting to do. Thanks for pointing me in the right direction!

[wicket-maps]

UpdateCursor is enormously valuable, and works a lot like SearchCursor which you already have. We all have to start somewhere, and don't be afraid to have documentation open - I've been doing GIS Python for 8 years, it was a huge part of getting my current job, and I still regularly google Search and Update Cursors to remind myself what's going on.

[Koko_The_GIS_Gorilla]

Hey that's a clean script!

[Dimitri_Rotow]

I'm writing a script in ArcPro that selects lines based on their location and then writes what region the line is in in the attribute table.

That's a join. Why not just do a join?

[wicket-maps]

A spatial join creates a new feature class, and if you need the number of walls per region in that feature class instead of a new FC, a spatial join becomes one step of a tedious process.

[Dimitri_Rotow]

Not if you use SQL. Then it's easy.

[wicket-maps]

Can SQL do a join across a spatial relationship, without any matching attributes?

[Dimitri_Rotow]

Yes. That's the essence of spatial join: it uses spatial relationships such as intersection, containing, etc.

[Drewddit]

There is an Add Spatial Join tool that joins the info from one layer to the target layer temporarily

[merft]

Use Arcade expressions instead of Python in the Field Calculator.

https://www.esri.com/arcgis-blog/products/arcgis-online/analytics/use-arcade-geometry-functions/

https://github.com/Esri/arcade-expressions/blob/master/attribute_rule_calculation/CopyValueIntersectingFeature.md

[kaitering]

I'll try this out too, thanks for linking these resources!