I am trying to take the population census tract and calculate the percentage of population by each census tract in the field (Populati_2). To do this I'm creating a new field with the total sum of the population (pop).

To get the percentage I'm usingthe following expression on arcgis [Populati_2] / [Pop] *100. It works fine in arcgis but when I'm using it to calculate field management or python it just doesn't work.

I'm not sure how to format the expression, I have tried every single combination and it's just not working.

Here's part of the script:

arcpy.AddField_management(r"Results\popjoin.dbf", "Pop", "LONG", "", "", "5")
arcpy.AddField_management(r"Results\popjoin.dbf", "Pop2", "DOUBLE", "", "", "5")
arcpy.CalculateField_management(r"Results\popjoin.dbf", "Pop", summed_total)
arcpy.CalculateField_management(r"Results\popjoin.dbf", "Pop2", '''Populati_2] / [Pop] *100''')

This one gives me errors, but the following does not give me any errors but I get a 0 in every row instead of the calculation.

expression = "!Populati_2! / !Pop! * 100"

arcpy.CalculateField_management(r"Results\popjoin.dbf", "Pop2", expression, "PYTHON_9.3")

2 Answers 2


As you can read in the commands, be careful what is a VBS and what a Python command/expression.

fc = "file_name"

# http://gis.stackexchange.com/a/82251/53268
summed_total = 0
with arcpy.da.SearchCursor(fc, "Populati_2") as cursor:
    for row in cursor:
        summed_total = summed_total + row[0]

arcpy.AddField_management(fc, "Pop", "LONG", "", "", "5")
arcpy.CalculateField_management(fc, "Pop", summed_total)

arcpy.AddField_management(fc, "Pop2", "DOUBLE", "", "", "5")
arcpy.CalculateField_management(fc, "Pop2", "!Populati_2! / !Pop! * 100", "PYTHON", "#")

If you have problems with the expression, you can always use the Field Calculate Toolbox to build your expression. See your old Result under Geoprocessing -> Results

arcgis result

With a double click on Calculate Field you can open the toolbox. Here you can use the expression builder (make sure you set Python as Parser):

calculate field


If you are unsure of the expression syntax, try the following:

  1. Crate a module in ModuleBuilder, add the Calculate Field tool, define all parameters, and run the model.
  2. If it runs successfully, export the model File>Export to python script
  3. Use the python expression syntax from the exported .py from step 2 into your python Calculate Field statement

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