Hi,

check whether your object is browsable, and you can call function by

env['pos.order'].cost_elab() like self.env['product.uom']._compute_price(line.product_id.uom_id.id, purchase_price, to_uom_id=line.product_uom.id)

class x(Object):

     def y():

         return "teste"

class y(models.Model, x):

     """ here I can access function y as self.y() """

     print self.y()

if still probs, then please give me your example code

In case of odoo lets take an example.

class res_partner(models.Model):
    _inherit = "res.partner"
    @api.multi
    def google_map_img(zoom=17, width=298, height=298):
        partner = self.browse([partner_id])
        params = {
        'center': '%s, %s %s, %s' % (partner.street or '',
        partner.city or '',
        partner.zip or '',
        partner.country_id and partner.country_id.name_get()[0][1] or ''),
        'size': "%sx%s" % (height, width),
        'zoom': zoom,
        'maptype': 'hybrid',
        'sensor': 'false',
         }
        return urlplus('//maps.googleapis.com/maps/api/staticmap', params)

class test(models.Model):
     _name = 'your.name'

     @api.multi
     def test(self):
     partner = self.sudo().partner_id
     return partner and partner.google_map_img(zoom, width, height) or None

In case you have no record set try @api.model instead of multi