ArcGIS Python Label Expression Error

I've recently started learning Python and started using it to create label expressions in ArcMap. I ran into an error today that I can't figure out though (and seems like it should be simple). When I try to create the following expression:

def FindLabel ( [FacilityName] ):
  S = [FacilityName] 
  S = S.upper()
  return S

I get back an error as follows:

Error 0 on line 0.
Error running expression:  FindLabel(ESRIExpressionArg0)
Traceback (most recent call last):
 File "<expression>", line 1, in <module>
 File "<string>", line 3, in FindLabel
AttributeError: 'NoneType' object has no attribute 'upper'.

[FacilityName] is a domained field, and Null values are allowed. I understand, I think, that 'NoneType' means that [FacilityName] is being given a None value before it's trying to be returned, but I don't know how to fix the problem.

Thanks,

Ethan

Solution

I'm pretty sure all the values get returned as strings, but in the case of a null value, it might be returned as None, which would explain the error you're getting.

You can use a try..except block or make a conditional statement inside your function to ignore None values.

Here's with try..except:

def FindLabel ( [FacilityName] ):
    try:
        S = [FacilityName] 
        S = S.upper()
        return S
    except AttributeError:
        pass