[web2py] Lazy virtual fields - strange result!

[Martin Weissenboeck]

I wanted to learn more about lazy virtual fields and therefore I have
repeated the example from the book:

def vfields():
   db.define_table('item',
     Field('unit_price','double'),
     Field('quantity','integer'))

   db(db.item.id>0).delete()

   class MyVirtualFields:
     def lazy_total_price(self):
       return lambda self=self: self.item.unit_price*self.item.quantity

   db.item.virtualfields.append (MyVirtualFields())

   db.item.bulk_insert([{'unit_price':12.00, 'quantity': 15},
     {'unit_price':10.00, 'quantity': 99},
     {'unit_price':120.00, 'quantity': 2},])

   res = []
   for r in db(db.item.id>0).select():
     res.append([r.unit_price, r.quantity, r.lazy_total_price()])
   return dict(res=res)

The expected output is:
  [[12.0, 15, 180.0], [10.0, 99, 990.0], [120.0, 2, 240.0]]

But I got
*  [[12.0, 15, *240.0]*, [10.0, 99, *240.0*], [120.0, 2, 240.0]]*
*
*
*Three times the same result.
*
I have read the book and my program over and over again - but I cannot see
any error.*
*

Does somebody have an idea?
Martin