Complicated expression for Calculate Field for arcpy

Question

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)"
arcpy.CalculateField_management("Layer","Field4",expression)

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

How can I write this expression?

Answer 1

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

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

0.3-.001*math.log(!Field1!+1)-0.02*math.log(!Field2!+1)-0.04*math.log(!Field3!+1)

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)'
 arcpy.CalculateField_management("Layer","Field4",expression,"PYTHON_9.3")