I want to add a computation in purchase.order.line using the existing onchange_product_id but I'm getting with an error of this:  TypeError: float() argument must be a string or a number I need help please i dont know what to do with this. here's my code:

 def onchange_product_id(self, cr, uid, ids, pricelist_id, product_id, qty, uom_id,
            partner_id, date_order=False, fiscal_position_id=False, date_planned=False,
            name=False, price_unit=False, rss_l = 0.0, rss_w = 0.0, b = 0.0, context=None):
        """
        onchange handler of product_id.
        """
        if context is None:
            context = {}
 
        res = {'value': {'price_unit': price_unit or 0.0, 'name': name or '', 'product_uom' : uom_id or False}}
        if not product_id:
            return res
 
        product_product = self.pool.get('product.product')
        product_uom = self.pool.get('product.uom')
        res_partner = self.pool.get('res.partner')
        product_supplierinfo = self.pool.get('product.supplierinfo')
        product_pricelist = self.pool.get('product.pricelist')
        account_fiscal_position = self.pool.get('account.fiscal.position')
        account_tax = self.pool.get('account.tax')
 
        # - check for the presence of partner_id and pricelist_id
        #if not partner_id:
        #    raise osv.except_osv(_('No Partner!'), _('Select a partner in purchase order to choose a product.'))
        #if not pricelist_id:
        #    raise osv.except_osv(_('No Pricelist !'), _('Select a price list in the purchase order form before choosing a product.'))
 
        # - determine name and notes based on product in partner lang.
        context_partner = context.copy()
        if partner_id:
            lang = res_partner.browse(cr, uid, partner_id).lang
            context_partner.update( {'lang': lang, 'partner_id': partner_id} )
        product = product_product.browse(cr, uid, product_id, context=context_partner)
        name = product.name
        if product.description_purchase:
            name += '\n' + product.description_purchase
        res['value'].update({'name': name})
 
        # - set a domain on product_uom
        res['domain'] = {'product_uom': [('category_id','=',product.uom_id.category_id.id)]}
 
        # - check that uom and product uom belong to the same category
        product_uom_po_id = product.uom_po_id.id
        if not uom_id:
            uom_id = product_uom_po_id
 
        if product.uom_id.category_id.id != product_uom.browse(cr, uid, uom_id, context=context).category_id.id:
            if self._check_product_uom_group(cr, uid, context=context):
                res['warning'] = {'title': _('Warning!'), 'message': _('Selected Unit of Measure does not belong to the same category as the product Unit of Measure.')}
            uom_id = product_uom_po_id
 
        res['value'].update({'product_uom': uom_id})
 
        # - determine product_qty and date_planned based on seller info
        if not date_order:
            date_order = fields.date.context_today(self,cr,uid,context=context)
 
 
        supplierinfo = False
        for supplier in product.seller_ids:
            if partner_id and (supplier.name.id == partner_id):
                supplierinfo = supplier
                if supplierinfo.product_uom.id != uom_id:
                    res['warning'] = {'title': _('Warning!'), 'message': _('The selected supplier only sells this product by %s') % supplierinfo.product_uom.name }
                min_qty = product_uom._compute_qty(cr, uid, supplierinfo.product_uom.id, supplierinfo.min_qty, to_uom_id=uom_id)
                if (qty or 0.0) < min_qty: # If the supplier quantity is greater than entered from user, set minimal.
                    if qty:
                        res['warning'] = {'title': _('Warning!'), 'message': _('The selected supplier has a minimal quantity set to %s %s, you should not purchase less.') % (supplierinfo.min_qty, supplierinfo.product_uom.name)}
                    qty = min_qty
        dt = self._get_date_planned(cr, uid, supplierinfo, date_order, context=context).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        qty = qty or 1.0
        res['value'].update({'date_planned': date_planned or dt})
        if qty:
            res['value'].update({'product_qty': qty})
 
        # - determine price_unit and taxes_id
        if pricelist_id:
            price = product_pricelist.price_get(cr, uid, [pricelist_id],
                    product.id, qty or 1.0, partner_id or False, {'uom': uom_id, 'date': date_order})[pricelist_id]
        else:
            price = product.standard_price
 
        taxes = account_tax.browse(cr, uid, map(lambda x: x.id, product.supplier_taxes_id))
        fpos = fiscal_position_id and account_fiscal_position.browse(cr, uid, fiscal_position_id, context=context) or False
        taxes_ids = account_fiscal_position.map_tax(cr, uid, fpos, taxes)
        res['value'].update({'price_unit': price, 'taxes_id': taxes_ids})
          
          
        l = 0.0
        w = 0.0
          
        if float(rss_l) >= 1:
             l = float(rss_l) / 25.40
          
        if rss_w :
             w = rss_w / 25.40
              
          
        a = 0.00
        m = 0.00   
        if w > 0 and l > 0 and b > 0:
            n = (round(w, 2) * round(l,2) * round(b,2))
            a = 1000000.00 / (n/ 1550.00 )
            m = a / qty
 
          
          
        res['value'].update({'rss_li': l,
                             'rss_wi': w,
                             'metric_tons':m})
          
          
        return res