Am I over-complicating adding and then calculating fields? Suggestions for a better way? (Python)

Question

I'm attempting to re-position a question I asked yesterday here. I am starting to think that the way I wrote my code just isn't the best way. Am I overcomplicating things?

I know for a fact that adding a bunch of new fields and then calculating those fields based on the new fields is a common occurrence in GIS, so I just can't understand why it's so difficult to find out how to do it in a Python script. The following code is a snipet of what I came up with, but despite hours and hours of troubleshooting, I keep getting an error (although it did work on a small sample size). I'll paste the code first, and then the error below.

I'm receiving the error just after all of the fields are successfully added. I've done a number of things in an effort to troubleshoot, such as import all of my files into a geodatabase and run it from there; I've tested small samples of data (which does actually work, so I know the code "works"), and I've switched out fields I thought were problematic with other fields that were not. So far, I'm unable to run this code on all 108 fields that I'm trying to add and then calculate. Again, I receive an error just before it attempts to calculate the fields, and it seems to be referring to the line: value = row.getValue(field)* row.anotherField

Any suggestions for a better way to do this??

Code snipet:

....

fc = arcpy.CopyFeatures_management(layerName, outFeatures)

#---------------------------------------------
# Add fields to a feature class:
#---------------------------------------------
fieldList = ["field1", "field2", "field3", ............., "field99"]  #<-- fieldList
fieldListx = []
for f in fieldList:
    fieldx = f+"x"
    arcpy.AddField_management(fc, fieldx, "DOUBLE")
    fieldListx.append(fieldx)

#---------------------------------------------
# Calculate fields:
#---------------------------------------------
rows = arcpy.UpdateCursor(fc)
for row in rows:
    for fieldx in fieldListx:
        field = fieldx.rstrip('x')
        value = row.getValue(field)* row.anotherField  #<--This is the line the error is referring to
        row.setValue(fieldx,value)
    rows.updateRow(row)
del row, rows

Error:

 Traceback (most recent call last):
   File "C:\GIS\1GIS_DATA\CUR_GIS\Election_Districts\Python and Tools\Join_Voter_Results_to_Allocation_Layer_and_Add_New_Fields.py", line 125, in <module>
     value = row.getValue(field)* row.UNpctOfEd #<--STOPS WORKING WHEN I ADD THE LARGER LIST!!!
   File "C:\Program Files\ArcGIS\Desktop10.0\arcpy\arcpy\arcobjects\arcobjects.py", line 944, in getValue
     return convertArcObjectToPythonObject(self._arc_object.GetValue(*gp_fixargs(args)))
 RuntimeError: ERROR 999999: Error executing function.

Answer 1

With respect to the line throwing the error: value = row.getValue(field)* row.UNpctOfEd maybe check the data type or value in each of the f and UNpctOfEd fields? Check note 1 below.

If all input fields are similarly named, you might also want to generate your fieldList with the List Fields function instead of typing it out:

# list fields for calculations
wild_card = "field*"
listFields = arcpy.ListFields (fc, wild_card, "DOUBLE")

Use the field_type parameter if you know your fields of interest are only of one data type - in this example "DOUBLE" (if not all the same then leave this out; if a given field is not numeric then see note 1 below).

Unless I'm missing something here, you shouldn't need to perform the field calculation inside a cursor (unless perhaps you need to accumulate values or similar) nor go through a second loop. Why not use the Calculate Field tool directly inside the first loop?

# loop through fieldList    
for f in fieldList:
    # local variables
    fieldx = f + "x"
    expression = "!" + f + "! * !" + anotherField + "!"
    # add/calculate new field
    arcpy.AddField_management(fc, fieldx, "DOUBLE")
    arcpy.CalculateField_management(fc, fieldx, expression, "PYTHON_9.3")

Notes:

1. If you know that your values may be stored as string then you will need to cast as numeric; e.g. replace f with float(f) in the expression variable. This is related to the comment from blah238.