I am trying to add a new field (float data type) and using existing fields to calculate the new field.

So far, I have:

import arcpy
path = 'pathname'  #in my code, this is actually the path name
arcpy.env.workspace = path

fclist = arcpy.ListFeatureClasses()
fc = fclist[1]

prop = 'PrYouth'  #name of new field
ftype = "FLOAT"  #Type of new field

thisField = arcpy.ListFields(fc, prop)
if len(thisField) == 0: #if length = 0 aka it does not exist yet, add field
    arcpy.AddField_management(fc, prop, ftype, "5", "4")  # 5 is precision, 4 is scale
else: #delete field then add new again
    arcpy.DeleteField_management(fc, prop)
    arcpy.AddField_management(fc, prop, ftype, "5", "4")  # 5 is precision, 4 is scale

with arcpy.da.UpdateCursor(fc, ['AGE_UNDER5','AGE_5_17','POP2000', prop]) as cursor:
    for row in cursor:
        kids = row[0] + row [1]
        row[3] = (kids/row[2])

I am unable to update the precision and scale values for float. I've tried to specify using field_precision = 5 and field_scale = 4, but this doesn't seem to work.

Also, how do I get the new field to calculate for the proportion? Am I using the right syntax?

Currently, I am only able to add the field and fill the list with 0.

  • Which versions of ArcGIS do you have? – Shiuli Pervin Sep 29 '16 at 9:38

When defining the precision and scale, its expecting an integer, you are currently passing it a string. Change to:

arcpy.AddField_management(fc, prop, ftype, 5, 4)

For the calculation, what are the datatypes of the other fields? Are you performing integer division? If so you need to cast to float first.

Per the doc's, these are ignored if its a personal or file gdb.


  • I did that at first, but it still didn't work. I tried to do it in model builder, and that was the way the exported python script looked like. – Leticia Sep 28 '16 at 18:06
  • Casting as a float first made it work! Thank you! :) – Leticia Sep 28 '16 at 18:11

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