Stuck updating one field based on the condition of another using Acrpy or Field Calculator

Question

I have a shapefile with 5,000 city points. I need to populate a new field, "cls_pop", based on each cities population, so that cities with a population > 2,000,000 will have "cls_pop" = 1, cites with a population > 1,800,000 and < 1,600,000 = 2 and so on and so on. I know there are a lot of different ways to accomplish this and I think I have really confused myself reading to much. I want to write this as a arcpy script so I can change out the variables and use it on different fields but I'm unsure of the best way to go about this. I wrote this but it just places "1" in each row of pop_class.

city_points = "C:\\Desktop\\citypoints.shp"

cursor = arcpy.SearchCursor(city_points,"", "", "population")

for row in cursor:
    if row.getValue(cursor) > 2000000:
        arcpy.CalculateField_management(city_points, "cls_pop", '"{0}"'.format(1), "PYTHON")

    if row.getValue(cursor) > 1800000 and row.getValue(field) < 2000000:
        arcpy.CalculateField_management(city_points, "cls_pop", '"{0}"'.format(2), "PYTHON")

    ect……………….

Am I on the correct track or is there a better way for me to do this?

Answer 1

This code is untested but I think will be much closer to what you want - key first thing is that you need to use an UpdateCursor.

The other thing that I have done is to use three ways to get and set the values of the current row in your cursor - in practice just choose one of them.

The first (on the if) is the simplest but "hardwires" the field names.
The second (on the elif) uses a slightly longer syntax but still "hardwires" the field names.
The last (on the else) lets you pass in a variable holding the field name.

city_points = "C:\\Desktop\\citypoints.shp"

cursor = arcpy.UpdateCursor(city_points)
classPopVar = "cls_pop"

for row in cursor:
    if row.population >= 2000000:
        row.cls_pop = 1
    elif row.getValue("population") >= 1800000 and row.getValue("population") < 2000000:
        row.setValue("cls_pop") = 2
    else:
        row.setValue(classPopVar) = 3
del row,cursor