Specifying field value in ArcPy update cursor for field calculation?

Question

I am trying to update a field using the arcpy.da.Update cursor where I can use the value from a select by location result to populate a new field. Ideally, I would like to be able to use a field value, GEN_UNITS, from a row selected by location from "JoinLayer" divided by "Lines" from the 'subLayer' feature class. What do I need to do to approach this correctly?

#Make feature layer
arcpy.MakeFeatureLayer_management(pp_trans_GUJoin, "JoinLayer")


with arcpy.da.UpdateCursor (subLayer, [GUCountField, "SHAPE@"]) as GUCalcCursor:
    for subrow in GUCalcCursor:
        arcpy.SelectLayerByLocation_management(
                "JoinLayer", 'WITHIN_A_DISTANCE', 
                subrow[1], .000002, "NEW_SELECTION") 
        result = pp_trans_GUJoin.GEN_UNITS/subLayer.LINES 
        subrow[0] = result
        GUCountField.updateRow(subrow)

print "GU_Count Updated"
del GUCalcCursor

Update* It seems like a more robust spatial join route may be the better approach here. Though I still need to use the update cursor to update the GC_Count field in the 'SubLayer' feature class. With this code, I am now receiving an error of 'tuple' object has no attribute 'GEN_UNITS' for the 'result = joinsubrow.GEN_UNITS/joinsubrow.LINES' line. Do I need to search out these field values separately in separate search cursors? What would be the best route here?

import arcpy
from arcpy import env

arcpy.env.OverwriteOutput = True

defaultGdbPath = 'C:\Topo_Check_Tess_V5.gdb'

subLayer='C:\Topo_Check_Tess_V5.gdb\Subs'
transLayer='C:\Topo_Check_Tess_V5.gdb\TLines'
ppLayer='C:\Topo_Check_Tess_V5.gdb\PPs'


arcpy.AddField_management(subLayer, "GU_Count", "SHORT", "", "", "", "", 
"NULLABLE", "NON_REQUIRED", "")
GUCountField = "GU_Count"



#Make feature layer
arcpy.MakeFeatureLayer_management(ppLayer, "ppLayerGU")

#Make feature layer
arcpy.MakeFeatureLayer_management(transLayer, "transLayerGU")

pp_trans_GUJoin ='C:\Topo_Check_Tess_V5.gdb\pp_trans_GU_SpatialJoin'

SubLayer_pp_trans_GUJoin = 
'C:\Topo_Check_Tess_V5.gdb\SubLayer_pp_trans_GUSpatialJoin'

#spatial join of feature layers
arcpy.SpatialJoin_analysis ("transLayerGU", "ppLayerGU", pp_trans_GUJoin, 
'JOIN_ONE_TO_ONE', 'KEEP_ALL', '#', 'WITHIN_A_DISTANCE', .000002) 

#Make feature layer
arcpy.MakeFeatureLayer_management(subLayer, "SubLayer_Layer")

#spatial join of feature layers
arcpy.SpatialJoin_analysis ("SubLayer_Layer", "ppLayerGU", 
SubLayer_pp_trans_GUJoin, 'JOIN_ONE_TO_ONE', 'KEEP_ALL', '#', 
'WITHIN_A_DISTANCE', .000002) 


#Make feature layer
arcpy.MakeFeatureLayer_management(SubLayer_pp_trans_GUJoin, "JoinLayer")

LINE_Fields = ['LINES','GEN_UNITS']


with arcpy.da.UpdateCursor (subLayer, [GUCountField, "SHAPE@"]) as 
GUCalcCursor:
    for subrow in GUCalcCursor:
        arcpy.SelectLayerByLocation_management("JoinLayer", 
        'WITHIN_A_DISTANCE', subrow[1], .000002, "NEW_SELECTION") 

        with arcpy.da.SearchCursor("JoinLayer", LINE_Fields) as 
        JoinLayerCursor:
            for joinsubrow in JoinLayerCursor:

                result = joinsubrow.GEN_UNITS/joinsubrow.LINES 

    subrow[0] = result

    GUCountField.updateRow(subrow)

Answer 1

I Think a spatial join might be better but anyway this should work (i might be mixing up your field names):

import arcpy

arcpy.MakeFeatureLayer_management(pp_trans_GUJoin, "JoinLayer")
subrowcount=1
with arcpy.da.UpdateCursor (subLayer, [GUCountField, "SHAPE@","LINES"]) as GUCalcCursor:
    for subrow in GUCalcCursor:
        arcpy.SelectLayerByLocation_management("JoinLayer", 'WITHIN_A_DISTANCE', subrow[1], .000002, "NEW_SELECTION")
        #Make sure only one feature is selected:
        if int(arcpy.GetCount_management(in_rows="JoinLayer").getOutput(0))==1:
            #From the selected feature get the GEN_UNITS value
            with arcpy.da.SearchCursor("JoinLayer","GEN_UNITS") as cursor:
                for row in cursor:
                    GEN_UNITS_value=row[0]
            result = GEN_UNITS_value/subrow[2]
            subrow[0] = result
            GUCalcCursor.updateRow(subrow)
        else:
            print 'Failed to update subrow {0}'.format(subrowcount)
        subrowcount+=1
print "GU_Count Updated"