ArcGIS Pro 3.1.7 - Calculate Fields geoprocessing not using expression!

Question

Hi,

I have a simple script using CalculateFields geoprocessing tool. As part of my script, I build an expression in a loop for fields to calculate. However, it is not working...

If I use hard-coded field names and expression for calculation, it works.

Why my 'expression' string variable is not working? It is exactly the same as the hard-coded string...

arcpy.management.CalculateFields(in_table=fc, expression_type="PYTHON3", fields=[["Detached", "!Detached!*!Ratio!", ""], ["Total", "!Total!*!Ratio!", ""]])[0]

arcpy.management.CalculateFields(in_table=fc, expression_type="PYTHON3", fields=expression)[0]

VincentLaunstorfer · Answer

Thanks for looking at this post.

The content of the expression variable is the second line of the code, as below:

[["Detached", "!Detached!*!Ratio!", ""], ["Total", "!Total!*!Ratio!", ""]]

As you mentioned, the CalculateFields is expecting a Value table and here I tried to built a string. In fact, I tried to reuse the syntax provided when you export ModelBuilder to python... hence the [0] which I do not either understand its use.

As for error message, on runtime, Python simply ignore the CalculateFields!

As I said, using arcpy.management.CalculateField in a loop is an excellent workaround, using simple string variable (no need for Value table)

GIS-Chops · Answer

It would be helpful if you posted the contents of the expression variable and any errors that come up when attempting to run the tool.

The tool is expecting a Value Table which is essentially an array of arrays. If you enclosed your value table in quotes it isn't getting the proper input. I am also wondering what the [0] at the end of each line is for?

I'm thinking this would work:

arcpy.management.CalculateFields(in_table=fc, expression_type="PYTHON3", fields=[["Detached", "!Detached!*!Ratio!", ""], ["Total", "!Total!*!Ratio!", ""]])[0]
expression = [["Detached", "!Detached!*!Ratio!", ""], ["Total", "!Total!*!Ratio!", ""]]
arcpy.management.CalculateFields(in_table=fc, expression_type="PYTHON3", fields=expression)[0]

VincentLaunstorfer · Answer

I found a workaround... Instead of dealing with building an 'expression' string variable in a for loop to calculate all fields with CalculateFields, I am calculating fields directly in the for loop, one by one with  arcpy.management.CalculateField. It is just as good and the syntax is much easier!