Using arcpy update cursor to populate a field based on subtraction from two other fields

Question

In a counties shapefile, I have two fields named population2001 and population2010. I would like to create a third field with the difference between 2001 and 2010 using arcpy and the update cursor to better learn how to use cursors. My code:

   import arcpy
   arcpy.env.workspace =          
   r"C:\Users\Documents\geodatabase.gdb"
   ndCounties = r"C:\Users\Documents\geodatabase.gdb\Counties"
   #add the new field
   arcpy.AddField_management(ndCounties, "difference", "LONG", field_length = 10)

   fields = ['POP2001', 'POP2010', 'difference']

   with arcpy.da.UpdateCursor(ndCounties, fields) as cursor:
      for row in cursor:
         if row[0] == 'POP2001':
            row[1] = 1
            row[2] = 2
         elif row[0] == 'POP2010':
            row[1] = 2
            row[2] = 3
         row[3] = row[1] - row[2]
         cursor.updateRow(row)

When I run this code, I get this error:

TypeError: unsupported operand type(s) for -: 'float' and 'NoneType'

I'm not exactly sure if this is the right way of doing this field subtraction.

Answer 1

In your cursor the row followed by the number references each field:

• row[0] references POP2001 field
• row[1] references POP2010 field
• row[2] references difference field

You do not have a fourth field to reference with row[3].

What you want to do is subtract row[0] from row[1] to get the difference to put into row[2].

for row in cursor:
    row[2] = abs(row[1] - row[0])
    cursor.updateRow(row)

The abs() gives an absolute value. If you want to see where population has gone down (a negative difference) remove the abs():

row[2] = row[1] - row[0]