SQLAlchemy - Session.commit in Session.query loop resets object __dict__

When iterating over session.Query object and calling update, I've noticed that the returned objects no longer have their __dict__'s populated.

E.g.

foos = session.query(Foo)
for foo in foos:
    print "{}".format(foo.__dict__)
    foo.somefield = "somevalue"
    session.add(foo)
    session.commit()  # The next print statement won't show anything

If any of the fields in foo are read during the iteration, though, all of the __dict__ fields become resolved.

E.g.

foos = session.query(Foo)
for foo in foos:
    # Reading any of the fields, causes all of the fields to be resolved.
    foo.somefield
    print "{}".format(foo.__dict__)
    foo.somefield = "somevalue"
    session.add(foo)
    session.commit()  # The next print statement won't show anything

Is there any way to have these values appear in __dict__ without needing to read them first?