How do you test for a NULL string field in the Field Calculator with Python?

Question

I am trying to perform an operation on a string field with a condition that it contains data. Referencing None with Python does not appear to work when the field is nullable and contains <Null> for its data. Am I missing something? I have tried len() and "<Null>" and "Null" as well. What do?

The code, for kicks:

def apt(addr,aptnum):
 if aptnum != None:
 a = addr.split(aptnum,1)[-1]
 return a
 else:
 return addr 

Update: It would appear that my problem is that field calculations are not processing on any rows where a null column is tested. That is outside the scope of this question, however, so I will leave it for the time being.

• whuber: Have you compared it to the null string ""?
• Jason Scheirer: Are you on the latest service pack?
• Nathanus: @Jason Scheirer No, we have not upgraded to SP2 yet.
• Jason Scheirer: @Nathanus Ok, I just recently did a bunch of fixes related to NULLs in field calculator and I'm pretty sure they made it into SP2.
• user6173: I have the same problem. I am trying to remplace <Null> in a field with 0. I add another field and then I use the codeblock. But nothing is happend. codeblock: """def reclass ( Neighbours2): if Neighbours2 == None: return 0""" Expression: "reclass (!Neighbours2!)" Thanks for the help.

Answer 1

I don't know what you are querying (eg shapefile, geodatabase), but have a look at http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00s50000002t000000.htm specifically the section on "The NULL keyword"

EDIT Adding a field to a shapefile, using the field calculator on this field, then using the following as the code block (Python parser)

def TestForNull(a_field):
 if not a_field:
 return "is null"

with TestForNull(!YourFieldNameHere!) in the expression line, results in "is null" being added to the field. You will of course have to modify to suit your purposes.

• Nathanus: This works in the field calculator Codeblock? I'm a bit lost as to how this will help me out.
• user681: did you try it? ie "myFieldNameHere" IS NULL where of course the field is your field. In theory, it should select the records with Null values. I don't have any shapefiles that have null records, so you will have to test it on your own data. Check the other options if you don't want the null records.
• blah238: SQL syntax does not work on the Field Calculator dialog (unless you are using a cursor in the code block which is not a very good idea).
• whuber: @blah Dan Patternson is not using SQL syntax. His solution is pure Python.
• Nathanus: This works well enough. Apparently my problem is that the field itself is not responding to field calculations. Joy.

Answer 2

Try just

if aptnum:

This tests for both null and zero-length strings.

Answer 3

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or you could use if aptnum is None:

Answer 4

This answer from PolyGeo worked for me: https://gis.stackexchange.com/a/81155/44980

.strip() removes all whitespace characters

def TestForNull(Field1):
 if Field1.strip() == "":
 return 'is null'
 else:
 return 'not null'

• pbreach: This should be the accepted answer. I spent a lot of time figuring out that an empty text field is actually " " instead of "".

Answer 5

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Try calling the codeblock as apt(!addr!,str(!aptnum!)).