ArcPy Calculate Field Management with Variable

Question

I'm trying to calculate multiple fields that all begin with "B01". This calculation consists of multiplying the fields by a field that contains a percentage ("TractPercent") and then dividing the results of that multiplication by a population field ("BufferPOP").

The problem I'm having is incorporating the variable associated with my "for loop" with my Python3 expression.

Here is my code:

import arcpy
### set workspaces
arcpy.env.workspace = r"C:\arcGIS_Shared\Python\Demographics.gdb"
relpath = r'C:\arcGIS_Shared\Python'
p = arcpy.mp.ArcGISProject(relpath + r'\Demographics.aprx')
m = p.listMaps('Map')[0]
layers = m.listLayers("DalyCity_HalfMile_Demographics") 
for layer in layers:
 print(layer)
 DemoFields = arcpy.ListFields(layer, "B01*", "ALL")
 for DemoField in DemoFields:
 arcpy.CalculateField_management(layer, DemoField, "(+DemoField+ * 
 !TractPercent!) / !BufferPOP!", "PYTHON3")
 print(layer.name + ' ' + DemoField + ' Calculated')

When I run the code this way I get the following error, but can't figure out what I need to change.

ERROR 000622: Failed to execute (Calculate Field). Parameters are not valid.
ERROR 000623: Invalid value type for parameter field

• SMiller: What's the format of the DemoField? What happens if you add some print statements - are you getting the values you expect?
• Matt A.: @smiller I added some print statements and they returned what I expected them to. As far as the format, the data type is "Double". Is that what you were asking for?
• Tom: @MattA., could you give us an example of what the print statement returned? And I think smiller meant to print the expression itself. Did you try testing the result directly in the Field Calculator, not using ArcPy?

Answer 1

Your expression shouldn't include variable names. You're writing:

"(+DemoField+ * !TractPercent!) / !BufferPOP!"

but it should be:

"(!{}! * !TractPercent!) / !BufferPOP!".format(DemoField.name)

• Bjorn: Shouldn't it be DemoField.name?
• Matt A.: I've tried both "(!{}! * !TractPercent!) / !BufferPOP!".format(DemoField) and "(!{}! * !TractPercent!) / !BufferPOP!".format(DemoField.name), but I am still getting the same errors. I looked at the data types associated with the CalculateField_management, (pro.arcgis.com/en/pro-app/tool-reference/data-management/…) but didn't see anything that stood out. Except for maybe that .format(DemoField) is outside the quotes
• Tom: Thanks, Bjorn. I always get a list of names with list comprehension, so I overlooked that. It's fixed. @MattA., it's supposed to be outside the quotes. Otherwise, you're simply looking for a field that matches your variable name rather than the value held by the variable.
• Tom: @MattA., what are the datatypes of each of your fields?
• Matt A.: @Tom I tried running the code in the Code Block of the Field Calculator and got a similar error about a missing or incorrect parameter. The datatypes for all the fields I'm using in the code is "Double". As far as the print statements, I was able to successfully print the the Table name and the DemoField name. I'm not sure how I would print the expression, unless I assigned it to a variable.