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I want to use the field calculator to get the sum of a couple of columns. The problem is that some of them has Null Values. I am not able to find a solution. I tried this Python functions:

stack( !basic.Blue_Thickness!, !basic.Green_Thickness!, !basic.Red_Thickness!, !basic.F1_Thickness! )

Version 1:

def stack(*args):
  myList = list(args)
  myList = [item for item in myList if (item is not None)]
  return sum(myList)

Version 2:

def stack(*args):
    return sum(filter(None, args))

Version 3: 

def stack(*args):
  myList = list(args)    
  for item in myList:
    if item is None: 
      myList.remove(item)
  return sum(myList)

The result of all this functions is that I get always NULL back, only if there is a row with no NULL then I get a result.

Background: I have a File Geodatabase with one main table and some tables which joins this table. The calulation takes place only in the main table (source columns and write column). I am using ArcGIS 10.0. The Source of these functions is this discussion: Calculating field in which null values may be present?

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4
  • Have you tried 'item == None'?
    – PolyGeo ♦
    Jun 30, 2013 at 4:53
  • thank you unfortunately it didn't worked. i tried it two times: 'code' def stack(*args): myList = list(args) for item in myList: if item == None: myList.remove(item) return sum(myList) def stack(*args): myList = list(args) myList = [item for item in myList if (item != None)] return sum(myList)'code'
    – user19605
    Jun 30, 2013 at 6:24
  • I'll try and run a more complete test. Perhaps you can edit your question to include the additional test(s) you ran and the errors that resulted from each test.
    – PolyGeo ♦
    Jun 30, 2013 at 6:59
  • just a general python note: if item is the same as if (item is not None) and easier to read. Likewise if not item is the same as if item == None. I'm not sure if this has any bearing on your Q as I can't test in Arc10 or earlier. FWIW, v1 returns extra Nulls in Arc10.3 also, v3 appears to work properly but beware because mutating a list in place causes unexpected but not immediately obvious mistakes in the results (see here). Bottom line: v2 works, don't use the others.
    – matt wilkie
    Jun 17, 2015 at 21:04

2 Answers 2

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I just ran the test below in the Field Calculator and it seemed to work.

def stack(item1,item2,item3):
  itemList = [item1,item2,item3]
  myList = [item for item in itemList if (item != None)]
  return sum(myList)

This was the Field Calculator settings I used:

enter image description here

and this is the test table with the result:

enter image description here

I think the only thing that I am not doing at the moment is joining through to another table but before I test that (I am using ArcGIS 10.1 SP1) perhaps you can confirm that this simple test is working for you. It's not as Pythonic but seems to work.

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There appears to be a problem with either earlier versions of Arc (I use 10.0) or some installations of Arc where anytime your Python script sees a <Null>, <Null> is returned. I am aware of the if myfield is None approach, but this did not work for me, or a commenter here, or several other users with similar problems in gis.stackexchange. No matter the various clever manipulations/if-elifs, the field is set to <Null> if a <Null> is encountered at all in the script.

The only solution that works for me is to use VB Script instead of Python. The isNull(myfield) function reliably checks if a field is <Null> without any of the problems above. It is not too difficult to translate simple Python to VB Script.

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