Why required and default are mutally exclusive in ndb?

Question

In old google appengine datastore API "required" and "default" could be used together for property definitions. Using ndb I get a

ValueError: repeated, required and default are mutally exclusive.

Sample code:

from google.appengine.ext import ndb
from google.appengine.ext import db

class NdbCounter(ndb.Model):
    # raises ValueError
    count = ndb.IntegerProperty(required=True, default=1)

class DbCounter(db.Model):
    # Doesn't raise ValueError
    count = db.IntegerProperty(required=True, default=1)

I want to instantiate a Counter without having to specify a value. I also want to avoid someone to override that value to None. The example above is constructed. I could probably live without a required attribute and instead add an increment() method. Still I don't see the reason why required and default are mutually exclusive.

Is it a bug or a feature?

Answer 1

I think you are right. Perhaps I was confused when I write that part of the code. It makes sense that "required=True" means "do not allow writing the value None" so it should be possible to combine this with a default value. Please file a feature request in the NDB tracker: http://code.google.com/p/appengine-ndb-experiment/issues/list

Note that for repeated properties things are more complicated, to repeated will probably continue to be incompatible with either required or default, even if the above feature is implemented.