Using select by attributes then calculate field on selection in ArcPy?

Question

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?

Answer 1

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'
        cursor.updateRow(row)

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', 
                                 where_clause=sql)
...