How to SUM field and then insert into NULL column?

Question

I am only in the beginning stages of learning Python.

I have a column, 'QUANTITY_SOLID', that I want to take the sum of all of its rows. I understand I can do this with 'SUMMARY_STATISTICS', but i'm not trying to create another entire table nor do anything other than simply SUM up that column. Once I have it summed up, I want to insert the sum into a new NULL field that I have created - I am attempting to do this via the 'CALCULATE_FIELD' tool.

QUANTITY_SOLID

QUANTITY_SOLID_SUM

CALCULATE_FIELD

• Aaron: Are you looking for a Python scripting solution or do you need to do this entirely in the field calculator?
• JumpInTheFire: @ Aaron, Honestly either would be fine. I'm trying to learn as much Python as I can but learning how to do it either way would be great.
• Michael Stimson: I think JumpinInTheFire is appropriate! You need to use a cursor, in this case an arcpy.da.searchcursor and arcpy.da.updatecursor. Google that and if you have difficulty understanding them I'll provide some code.

Answer 1

Cursors are the way to go for this type of problem.

First, create a list of values using a Search Cursor and a generator expression:

b = sum(row[0] for row in arcpy.da.SearchCursor(fc, 'QUANTITY_SOLID'))

Then run an Update Cursor to populate the QUANTITY_SOLID_SUM field:

with arcpy.da.UpdateCursor(fc, ['QUANTITY_SOLID_SUM']) as cursor:
 for row in cursor:

Finally, update the rows with the value from the generator

 row[0] = b
 cursor.updateRow(row)

---

import arcpy
# Define the feature class
fc = r'C:\path\to\your\fc'
# Use a generator expression to populate a list from the 'QUANTITY_SOLID' field
b = sum(row[0] for row in arcpy.da.SearchCursor(fc, 'QUANTITY_SOLID'))
with arcpy.da.UpdateCursor(fc, ['QUANTITY_SOLID_SUM']) as cursor:
 for row in cursor:
 row[0] = b
 cursor.updateRow(row)

• Michael Stimson: I like that Aaron for the simplicity! Please note JumpinInTheFire that that is not a field calculator expression.
• JumpInTheFire: Awesome, thank you both for the help, I greatly appreciate it. I didn't intend for you to write the code (just to push me in the right direction), but either way thanks!
• Paul: There's no need to sum up the values each iteration; remove it from the for loop. For that matter, you can pass a generator expression directly into sum, a la summed = sum(row[0] for row in arcpy.da.SearchCursor(fc, 'QUANTITY_SOLID'))
• Aaron: Good call @Paul. Post updated to address comments.