Calculate Field using Fieldnames(variables?!)

Question

the Question I have is kind of asked before. But I couldn't figure out the solution.

ArcGIS 10.2.2 Python 2.7.5

I`m adding fields to a table using user input for the fieldnames. Then I populate those fields and calculate with these fields yet another field.

It looks like this:

fieldname1 = arcpy.GetParameterAsText(2)
fieldname2 = arcpy.GetParameterAsText(3)

arcpy.AddField_management(FC, fieldname1, "SHORT","", "", "",
                          "", "NULLABLE")
arcpy.AddField_management(FC, fieldname2, "SHORT","", "", "",
                          "", "NULLABLE")

In the tool the datatype for the user input is set to "String".

Then I use search and update cursor to populate those fields. Afterwards I want to calculate the next field, so I build a codeblock and an expression and use CalculateField_management:

codeblock = """def myFunction(mthree, mtwo):
    if mthree >= 1 or mtwo >= 3:
        return -3
    elif mthree == 0 and (mtwo==1 or mtwo==2):
    return -2
    elif mthree == 0 and mtwo < 3 and mone >= 5:
    return -2
    if mthree == 0 and mtwo == 0 and mone ==4:
        return -1"""
expression = 'myFunction(fieldname1, fieldname2)'
arcpy.CalculateField_management(FC, "EPK_Gesamt",expression,"PYTHON_9.3",codeblock)

and that doesn't work. I tried several thing such as:

fieldn1 = '"!{}!"'.format(fieldname1)
fieldn2 = '"!{}!"'.format(fieldname2)
expression = 'myFunction(fieldn1, fieldn2)'

or:

expression = "myFunction('!' + fieldname1 + '!','!' + fieldname2 + '!')'

In both cases python returns me text values instead of fieldnames which doesn't work for the calculation in the calculatefield tool.

It works fine if I use the actual fieldnames in the expression variable:

expression = 'myFunction(!example1!, !example2!)'

But of course I want to use the user input, so the user doesnt have to work in the script. Anyone knows how to deal with that?

Answer 1

This should work for you:

import arcpy

fc = r"C:\Users\us\Documents\ArcGIS\Default.gdb\_PointDistanceFc"
field1 = arcpy.GetParameterAsText(0)
field2 = arcpy.GetParameterAsText(1)

codeblock = """def myFunction(field1_dum, field2_dum):
    if field1_dum >= 1 or field2_dum >= 3:
        return -3
    elif field1_dum == 0 and (field2_dum==1 or field2_dum==2):
        return -2
    elif field1_dum == 0 and field2_dum < 3 and mone >= 5:
        return -2
    if field1_dum == 0 and field2_dum == 0 and mone ==4:
        return -1"""

expression = 'myFunction(!{0}!, !{1}!)'.format(field1,field2)

arcpy.CalculateField_management(fc,"EPK_Gesamt",expression,"PYTHON_9.3",codeblock)

I've created a script tool with this code only and two input params of String types. The field names strings I submit are being inserted into the expression correctly.

As of personal preference, consider using the arcpy cursors for updating the fields instead of running the GP tool.

Answer 2

I have one more question regarding that issue:

>>> test1 = "test1"
>>> fieldn = '!{}!'.format(test1)
>>> print fieldn
!test1!


fieldna = '(!{0}!, !{1}!)'.format(test1,test2)
>>> print fieldna
(!test1!, !test2!)

Now I do see a difference. If I use the format funtion I get a tuple, but the format fits anyway. I don't get why I cannot use the first version in my function like

expression = 'myFunction(test1,test2)'

It works fine with :

expression = 'myFunction(!{0}!, !{1}!)'.format(test1,test2)

But I don't understand why