This resource shows how to use NULL values in PyQGIS. Within the Python Colsole the comparison value == NULL passes without exception. However, when run from my plugin NULL is neither in the global nor the local namespace:
NameError: global name 'NULL' is not defined
The only way to check for NULL values seems to be to check for an instance of QPyNullVariant:
from PyQt4.QtCore import QPyNullVariant
if isinstance(value, QPyNullVariant):
pass
How do I import NULL so I can compare value == NULL?
For those wondering why and how NULL is different from None have a look at this blog entry. Essentially historically is wasn't possible to return None:
Turns out by removing QVariant from PyQt it had some impact on methods that expected a NULL QVariant - A QVariant with no value. Passing None didn't work because those methods needed the type information that QVariant holds, even when NULL.
On None comparisons it says:
One way to check if something is Null in Python is to use value is None however this will not work with our NULL type. Overloading the is operator in Python is not supported and there is no way we can support this - trust me I have tried. is is really doing id(object) == id(object) under the hood:
