In OpenERP, create a custom module like: demo and then create file like: demo.py file in your demo module and demo.py file pass in your other file __init__.py file. And then create other file demo.xml file and pass this file in your other file like as a __openerp__.py file. Then create a function in your demo.py file and pass this function in the demo.xml file.

Using this code given below

def productid_change(self,cr,uid,ids, product ,uom_id,  qty=0, name='', type='out_invoice',
            partner_id=False, fposition_id=False, price_unit=False, currency_id=False,
            company_id=None,context=None):
        res = super(account_invoice_line, self).product_id_change(cr,uid,ids,product, uom_id, qty, name,type, partner_id, fposition_id,price_unit, currency_id,company_id,context)
        if res['value'].get('name', False):
            name = res['value'].get('name', False)
            name = name.split(" ")
            prod_name=name[0]
            prod_name=prod_name.replace("["," ")
            prod_name=prod_name.replace("]"," ")
            prod_name=prod_name.strip()
            result=res['value'] 
            product_obj=self.pool.get('product.product')
            var=product_obj.search(cr, uid, [('default_code', '=',prod_name)])
            product_obj1 = product_obj.browse(cr, uid, var, context=context)
            prod_type = product_obj1.supp_ref
            prod_price = product_obj1.lst_price
            squ_meter = product_obj1.squ_meter
            subtotal = (qty) * (prod_price)
            result['price_subtotal'] = subtotal
            result['qty_char'] = qty * squ_meter
            return {'value' : result}

        return res