Empty list in App Engine Datastore: Java vs Python

Question

I have the following java model class in App Engine:

public class Xyz ... {
    @Persistent
    private Set<Long> uvw;
}

When saving an object Xyz with an empty set uvw in Java, I get a "null" field (as listed in the appengine datastore viewer). When I try to load the same object in Python (through remote_api), as defined by the following python model class:

class Xyz(db.Model):
    uvw = db.ListProperty(int)

I get a "BadValueError: Property uvw is required".

When saving another object of the same class in Python with an empty uvw list, the Datastore viewer prints a "missing" field.

Apparently empty lists storage handling differs between Java and Python and lead to "incompatible" objects.

Thus my question: Is there a way to, either:

• force Java to store an empty list as a "missing" field,
• force Python to gracefully accept a "null" list as an empty list when loading the object?

Or any other suggestion on how to handle empty list field in both languages.

Thanks for your answers!

Answer 1

It should work if you assign a default value to your Python property:

uvw = db.ListProperty(int, default=[])