Question

I am new with developing web applications with Google App Engine.

I wanted to check for entities in my datastore which have a null value set for a list property (db.ListProperty). However, when I tried to check anything against the entity.list GAE gives the error:

'super' object has no attribute 'list'.

After some search I saw in this SO question that setting an empty value for a list property of a GAE datastore entity is equivalent to not setting the property at all. This explains my error.

So I need to match entities in the datastore which do not have any list property set at all. Looking at the GAE documentation I still haven’t found any method which allows me to check whether an entity has a particular property set.

NOTE: I don’t have to do this with GQL. I can retrieve all entities with GQL, then check with python. But a GQL solution is also fine.

Answer 1

I can think of three ways around this.

You’re probably running into this situation because originally your model did not have a ‘list’ attribute, and you added one later, so you might have older instances in the datastore without the ‘list’ attribute. You can write a mapreduce function that goes through your datastore and ensures all entities have the ‘list’ element.

Otherwise, you can use the python hasattr function

if hasattr(entity, 'list'):
    a = entity.list[0] # or whatever operation you want

Thirdly, you can use an exception handler to catch the error case.