I have a working formula that I can put in the Field Calculator which I'd like to use in my python code:

.3-.001*Log ( [Field1]+1 )-.02*Log ( [Field2]+1 )-.04*Log ( [Field3]+1 )

My code:

import math
expression =".3-.001*Log(!Field1!+1)-.02*Log(!Field2!+1)-.04*Log(!Field3!+1)"

ExecuteError: ERROR 999999: Error executing function. Syntax error Failed to execute (CalculateField).

How can I write this expression?

  • expression =".3-.001*math.log(!Field1!+1)-.02*math.log(!Field2!+1)-.04*math.log(!Field3!+1)"
    – Bjorn
    Nov 20, 2017 at 18:51

1 Answer 1


You should change "Log" to "math.log" .

If you want to use your code in Field Calculator, just use formula in it:


In Python IDE:

You need to assign "PYTHON_9.3" to the last parameter of the "arcpy.CalculateField_management".

Don't forget import arcpy in the beginning of the code when you use the code in the Python IDE or console.

 import arcpy,math
 expression = '.3-.001*math.log(!Field1!+1)-.02*math.log(!Field2!+1)-.04*math.log(!Field3!+1)'
  • ive always wondered why you need import arcpy for field calculator... its inside arcmap you would think its imported by default (like the arcmap python terminal?)
    – NULL.Dude
    Nov 20, 2017 at 20:14
  • 2
    I supposed you use this code in the python IDE. OK i edited my answer. If you use this code in the field calculator you'll not need to use import arcpy
    – BBG_GIS
    Nov 20, 2017 at 20:16
  • @Geo.Dude, @wetland If you're using this code in the field calculator you wouldn't be calling arcpy.CalculateField_management()
    – Midavalo
    Nov 25, 2017 at 4:11
  • @Midavalo You'r right. I edited the answer. Thanks.
    – BBG_GIS
    Nov 25, 2017 at 4:54

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