Using 'if' function to set field value based on various other field values gives ERROR 999999?

Question

I am trying to populate a field with a value from another field in the same row, the condition I have set for this to take place is if a third field is equal to 0. In order to achieve this I have written a python script utilizing an 'if' statement and the getValue and setValue functions. I haven't used these functions before and I am having difficulty getting them to work properly. I tried various different lines of syntax but keep getting the same error:

Runtime error : ERROR 999999: Error executing function.

This has left me a little baffled as I am not sure where the problem lies. Anyway, here's the script:

import arcpy

joinFC = "C:\MyArcGIS\Link_tool\Join22.shp"

rows = arcpy.UpdateCursor(joinFC)
for row in rows:
    if row.getValue("Input") == 0:
        edrn_value = row.getValue("LCC_DRN_3")
        row.setValue("DRN_input", edrn_value)
        rows.updateRow(row)

del row, rows

In case it is not obvious I am using ArcGIS 10.0. The field types for the three fields are:

LCC_DRN_3 = String

Input = Long

DRN_input = String

Does anyone have any idea why this script is not working?

Answer 1

Try this

from arcpy import env
d=joinFC.Describe
env.extent=d.extent

Strange but true, if your shapefile is outside processing extent, field calculator will fail on simplest tasks, that have nothing to do with shape field

Answer 2

It turns out that the error 999999 was a result of the field I was trying to extract a value from. The attribute table I was working with was the result of a spatial join and the field I used to join the tables was called LCC_DRN_ID. Because you cannot have two identically named fields in the same table the joined features LCC_DRN_ID field became LCC_DRN_3. For whatever reason ArcGIS did not like me using this field, so when I copied the contents of the LCC_DRN_3 field into a newly added field (DRN_update) the following script worked (thanks to Sleep6 for pointing me in the right direction):

import arcpy

joinFC = "C:\MyArcGIS\Link_tool\Join22.shp"

rows = arcpy.UpdateCursor(joinFC)
for row in rows:
    if row.getValue("Input") == 0:
        edrn_value = row.getValue("DRN_update")
        row.setValue("DRN_input", edrn_value)
        rows.updateRow(row)

del row, rows 

Guess I'll have to work an Add Field tool into my overall script!