Try Below code ...

class sale_order_line(osv.Model):

_inherit='sale.order.line'

  def _get_line_numbers(self, cr, uid, ids, fields, arg, context=None):

"""Assign sequential numbers to line items, starting at 1"""

if context is None: context = {}

res = {}

line_num = 1

# Take the first line in the order, find the root PO, then iterate through all

# PO line items to sequence them properly

#

  first_line_rec = self.browse(cr, uid, ids, context=context)[0]

for line_rec in first_line_rec.order_id.order_line:

res[line_rec.id] = line_num

line_num += 1

return res

_columns={

'sequence': fields.integer('Sequence'),

# TODO: Consider making this a stored field to save on read time

'line_no': fields.function(_get_line_numbers, type='integer', string='Line Number'),

XMl-----------------

<record id="view_order_form_double_validation" model="ir.ui.view">

            <field name="name">sale.order.form</field>

            <field name="model">sale.order</field>

            <field name="inherit_id" ref="sale.view_order_form"/>

                   <field name="arch" type="xml">

<xpath expr="//field[@name='order_line']/tree/field[@name='product_id']" position="before">

                       <field name="sequence" widget="handle"/> 

            <field name="line_no" string="Line No"/>

                </xpath>

Please upvote   my answer if resolved your issue          

You should consider using the sequence in SO line. You need to create a sequence and sequence code.Go through the following links:

http://www.zbeanztech.com/blog/sequence-openerp

https://matiar.wordpress.com/2011/09/29/sequence-in-openerp/

http://pinakinnayi.blogspot.in/2012/05/auto-number-generate-in-openerp.html