from lxml import etree

import simplejson #If not installed, you have to install it by executing pip install simplejson

@api.model

def fields_view_get(self, view_id=None, view_type=False, toolbar=False, submenu=False):

        res = super(SaleOrder, self).fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar,         submenu=submenu)

        doc = etree.XML(res['arch'])

        if view_type == 'form':

            for node in doc.xpath("//field"):

                modifiers = simplejson.loads(node.get("modifiers"))

                if 'readonly' not in modifiers:

                    modifiers['readonly'] = [['invoice_status','=','invoiced']]

                else:

                    if type(modifiers['readonly']) != bool:

                        modifiers['readonly'].insert(0, '|')

                        modifiers['readonly'] += [['invoice_status','=','invoiced']]

                node.set('modifiers', simplejson.dumps(modifiers)) 

                res['arch'] = etree.tostring(doc)

        return re