problem-sale-active-ids-wizard

class ImprentaConfirmarStart(ModelSQL, ModelView):
    'Modelo Confirmar Venta'
    _name = 'sale_imprenta.confirmar.start'

    cantidad_confirmada = fields.Selection('obtener_cantidades'
        , 'Cantidad confirmada')

    def obtener_cantidades(self):
        import pdb; pdb.set_trace()
        # I would like to have active_ids so i can get the value of my sale to create the selection field.                                                                                                  # not working ..
        ventas_obj = Pool().get('sale.sale').browse(Transaction().context.get('active_ids'))

        # EXAMPLE of return
        return [ ('100', '100'), ('200', '200') ]

ImprentaConfirmarStart()

class ImprentaConfirmarSale(Wizard):
    'Wizard Confirmar Venta. Confirma la cantidad de venta'
    _name = 'sale_imprenta.confirmar'

    start = StateView('sale_imprenta.confirmar.start',
    'sale_imprenta.cantidad_confirmar_form', [
    Button('Cancelar', 'end', 'tryton-cancel'),
    Button('Aceptar', 'aceptar', 'tryton-ok', True),
    ])

    aceptar = StateTransition()

    def transition_aceptar(self, session):
        print "Transition Acepto Confirmacion"
        ventas_obj = Pool().get('sale.sale').browse(Transaction().context.get('active_ids'))

        for venta in ventas_obj:
            venta.write(venta.id, {'state':'confirmed'})

        return 'end'

ImprentaConfirmarSale()