Computed fields can't be edited because they are computed. Maybe just set a default: db.article.price_with_taxes.default = lambda r: r.price*(1+r.taxes)/100)
I would typically not advise creating a field in the database that is simply a computation of another field. I would probably use a Virtual Field [1] or just do the math in my application. [1] http://web2py.com/book/default/chapter/06#Virtual-Fields
