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.

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.

Answer 2

Try just

if aptnum:

This tests for both null and zero-length strings.

Answer 3

or you could use if aptnum is None:

Answer 4

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