If you want generate the barcode for product in odoo You will need the following kind of python code declaration in your .py file:

from openerp.osv import fields, orm 
from openerp.tools.translate import _ 

def isodd(x): 
    return bool(x % 2) 



class product_product(orm.Model): 
    _inherit = 'product.product' 

    _columns = { 
        'ean_sequence_id': fields.many2one('ir.sequence', 'Ean Sequence'), 
    } 

    def _get_ean_next_code(self, cr, uid, product, context=None): 
        if context is None: context = {} 
        sequence_obj = self.pool.get('ir.sequence') 
        ean = '' 
        if product.ean_sequence_id: 
            ean = sequence_obj.next_by_id(cr, uid, product.ean_sequence_id.id, context=context) 
        elif product.categ_id.ean_sequence_id: 
            ean = sequence_obj.next_by_id(cr, uid, product.categ_id.ean_sequence_id.id, context=context) 
        elif product.company_id and product.company_id.ean_sequence_id: 
            ean = sequence_obj.next_by_id(cr, uid, product.company_id.ean_sequence_id.id, context=context) 
        else: 
            ean = sequence_obj.next_by_code(cr, uid, 'product.ean13.code', context=context) 
        if len(ean) > 12: 
            raise orm.except_orm(_("Configuration Error!"), 
                 _("There next sequence is upper than 12 characters. This can't work." 
                   "You will have to redefine the sequence or create a new one")) 
        else: 
            ean = (len(ean[0:6]) == 6 and ean[0:6] or ean[0:6].ljust(6,'0')) + ean[6:].rjust(6,'0') 
        return ean 

    def _get_ean_key(self, code): 
        sum = 0 
        for i in range(12): 
            if isodd(i): 
                sum += 3 * int(code[i]) 
            else: 
                sum += int(code[i]) 
        key = (10 - sum % 10) % 10 
        return str(key) 

    def _generate_ean13_value(self, cr, uid, product, context=None): 
        ean13 = False 
        if context is None: context = {} 
        ean = self._get_ean_next_code(cr, uid, product, context=context) 
        if len(ean) != 12: 
            raise orm.except_orm(_("Configuration Error!"), 
                 _("This sequence is different than 12 characters. This can't work." 
                   "You will have to redefine the sequence or create a new one")) 
        key = self._get_ean_key(ean) 
        ean13 = ean + key 
        return ean13 

    def generate_ean13(self, cr, uid, ids, context=None): 
        if context is None: context = {} 
        product_ids = self.browse(cr, uid, ids, context=context) 
        for product in product_ids: 
            if product.ean13: 
                continue 
            ean13 = self._generate_ean13_value(cr, uid, product, context=context) 
            self.write(cr, uid, [product.id], { 
                        'ean13': ean13 
                    }, context=context) 
        return True 

    def create(self, cr, uid, vals, context=None): 
        if context is None: context = {} 
        id = super(product_product, self).create(cr, uid, vals, context=context) 
        if not vals.get('ean13'): 
            ean13 = self.generate_ean13(cr, uid, [id], context=context) 
        return id 

    def copy(self, cr, uid, id, default=None, context=None): 
        if default is None: default = {} 
        if context is None: context = {} 
        default['ean13'] = False 
        return super(product_product, self).copy(cr, uid, id, default=default, context=context) 

You can start printing barcode from any row and column on PDF.