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I'm trying to copy an attribute value from one field to another (sum of raster values), but I'm having issues in writing a functioning expression for a Calculate Field script. I'm adding a field called SUM and trying to copy the value from the field "SUM_" + filename[0:-4]. I have multiple files I am trying to do this on and the field names are different in each, which is why it is being written in this manner.

Here's the code I have so far:

for table in arcpy.ListTables():
    arcpy.AddField_management(table, "SUM", "DOUBLE")
    expression = ["SUM_" + str(filename[0:-4])]
    arcpy.CalculateField_management(table, "SUM", expression, "VB")

This doesn't work as I receive the following error: 

File "C:\Users\kellyj\Desktop\Projects\CostAnalysis\Route_Cost_Analysis.py", line 82, in <module>
    arcpy.CalculateField_management(table, "SUM", expressionvalue, "VB")
File "C:\Program Files (x86)\ArcGIS\Desktop10.2\arcpy\arcpy\management.py", line 3354, in CalculateField
    raise e
RuntimeError: Object: Error in executing tool

Any thoughts as to how I can fix this?

share|improve this question
    
Thanks @Aaron. The script now finishes without an error, but the new "SUM" field is 0 in all of the shapefiles. Am I pointing to the source field properly in the expression? –  user28939 Sep 8 '14 at 20:40
    
Your script is creating the SUM field as a numeric Double field and then trying to calculate a string into it. Numeric fields cannot have non-numeric strings calculated into them. The SUM field has to be a string with enough characters to hold the concatenated "SUM_" plus the longest base file name. –  Richard Fairhurst Sep 8 '14 at 20:56
    
The reason Aaron's code produced 0 is that the input was without any delimiters and was interpreted as a variable. An uninitialized variable would be interpreted as 0. –  Richard Fairhurst Sep 8 '14 at 21:08
    
Use expression = "[SUM_" + str(filename[0:-4]) + "]" to make a field name inside a string. When interpreted by the Field Calculator the string from Python is just a field name, not a string. –  Richard Fairhurst Sep 8 '14 at 21:11

2 Answers 2

up vote 3 down vote accepted

You have to have a square bracket surrounding the "SUM_" thing

Trying change

expression = ["SUM_" + str(filename[0:-4])]

to

expression = "[SUM_{0}]".format(str(filename[0:-4]))
share|improve this answer
    
This did the trick, thanks much! –  user28939 Sep 9 '14 at 13:40
up vote 2 down vote

The brackets have to be inside the string, since otherwise the brackets will create a Python list. When Python passes a string that has brackets inside of it as the expression, Python does not interpret the brackets, but the Field Calculator interprets the bracketed expression within the string as a field name. Use the code that follows:

for table in arcpy.ListTables():
    arcpy.AddField_management(table, "SUM", "DOUBLE")
    expression = "[SUM_{0}]".format(str(filename[0:-4]))
    arcpy.CalculateField_management(table, "SUM", expression, "VB")
share|improve this answer
    
you are right, I tought that he wanted a string, not the field value. –  radouxju Sep 9 '14 at 7:11

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