How can I check for null values in the Field Calculator using Python?

Answer

Starting at ArcGIS for Desktop 10.1, null values in an attribute table are returned as the string 'None' in the Field Calculator using Python. Knowing this, an if/elif statement can be used to find whether values are null or not. Here is an example script that checks if a field contains null values:

Expression:
findNulls(!fieldA!)

Expression Type:
PYTHON_9.3

Code Block:
def findNulls(fieldValue):
    if fieldValue is None:
        return "null values"
    elif fieldValue is not None:
        return "no nulls here"