ArcGIS Pro 2.3

I want to create a standalone script which runs outside of ArcGIS Pro (for automation purposes), which selects features (with blank attributes) in a feature class and runs calculate field on those selections.

The code below doesn't work. It updates all fields, not the selected ones.

import arcpy
arcpy.management.SelectLayerByAttribute(r"C:\Automation\Categories\Coal\Coal_automation.gdb\CLAIMS", "NEW_SELECTION", "Claim_Spend_in_£ = ''", None)

arcpy.management.CalculateField(r"C:\Automation\Categories\Coal\Coal_automation.gdb\CLAIMS", "Claim_Spend_in_£", '"Unrecorded"', "PYTHON3", None)
print ("Done")

How can I write the code to run calculate field on a selection?


Use da.UpdateCursor instead of Field Calculator, it is more versatile and easier to use and you can pass the sql statement to only update "selected" rows:

import arcpy

fc = r"C:\Automation\Categories\Coal\Coal_automation.gdb\CLAIMS"
fieldname = "Claim_Spend_in_£"

sql = """{0} = ''""".format(arcpy.AddFieldDelimiters(fc,fieldname))

with arcpy.da.UpdateCursor(fc,fieldname,sql) as cursor:
    for row in cursor:
        row[0] = 'Unrecorded'

If you want to use Field Calculator on a layer:

A selection cant be made on a feature class, it has to be a layer. You can create a layer with MakeFeatureLayer then pass 'lyr' to calculate field:

sql = """{0} = ''""".format(arcpy.AddFieldDelimiters(fc,fieldname))
arcpy.MakeFeatureLayer_management(in_features=fc, out_layer='lyr', 
  • 2
    Thanks. The UpdateCursor worked fine when run in a script. Just had to remove the layer from ArcGIS Pro to avoid a 'could not acquire a lock' error. But thanks! – Theo F Oct 12 '18 at 13:22
  • It should also work with the layer added to the map and code pasted into the Python window. Did you have an Active edit session? I often add the layer to map, select some features and then use updatecursor in python window to only update selected features. – BERA Oct 12 '18 at 13:26
  • yeah I had been editing the attributes earlier within ArcGIS Pro. So there was a conflict when trying to edit it via an external script. I'm sure it'd have worked fine in the python console. – Theo F Oct 12 '18 at 13:30

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