In Odoo 8.0 - Accounting - While I'm creating an invoice (manually) once you add a products etc it calculate TOTAL price but to update tax must click "UPDATE" button. (WHY!?)

I would like to remove this button and change code to calculate everything again automatically after quantity is changed or something ?

I already tried call method button_reset_tax and calculate_taxes but I can't make it work. So if anyone would like to help me ... solve this issue ?

@api.onchange

def onchange_quantity(self):

    self.button_reset_taxes()# calling 'button_reset_taxes'

    print 'Recalculating VAT ....'

    return True

but it won't work .... getting error:

Traceback (most recent call last):

 ....................

return self._browse(Environment(cr, uid, context or {}), ids)

File "/usr/lib/python2.7/dist-packages/openerp/api.py", line 713, in __new__

self.cr, self.uid, self.context = self.args = (cr, uid, frozendict(context))

TypeError: 'float' object is not iterable

Any ideas ?

any ideas why it won't work ?