Use a python expression in Field Calculator to sum each row across several columns

Question

this seems like it should be easy but I am new to python and am having trouble... I have four columns. I would like perform a field calculation that sums the values by row for three of the columns and inserts that sum into the associated row of the 4th column. In vb script is it very straight forward. I would like to know how to do it in python. ie.

Col1    Col2    Col3    Col4(SummedValues)
  1       3     1              5
  2       1     1              4

Here is what I have:

I have tried adding a for statement (for rows in !resuly!:) but it returns a parsing syntax error (Error 000989 Python Syntax Error: Parsing error Syntax Error: invalid syntax (line 2)).

I am in ArcMap V.10.4

I feel like it is an iteration issue- maybe I need to tell it to go through each row i just don't know how to do that correctly in the field calculator...

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Per Phillip and JR comments: Code updated to and it worked perfectly.

Answer 1

I see a few issues with the code you show:

1. Your indentation is off - all lines after the def ... line should be on the same indentation level.
2. You need to pass the fields you want to add to your function, not the field you want to populate. So your definition should be something like def addition(a, b, c):. Whatever you have in the second box is what gets populated into the field, in this case, the result of your function, its return value.
3. For a simple thing like this, you should be able to do this without even using the codeblock. Just type !ones! + !tws! + !threes! into the second field, and you can remove whatever is in the codeblock field. As long as all the fields you are adding are of numeric type, this will work.

Answer 2

Per @PhillipNagel and @J.R this code block worked.