Hi Mehjabin.
1- thanks for guidance, really means a lot.
2- while trying doing that. below is my sample code from (models.py) and module is loaded but function is not getting called. unable to see any log msg in terminal. few more suggestions will help me go through.(thanks in advance)

from odoo import models, fields, api, loglevels

class rhein_validator(models.Model):
_name = 'rhein_validator.rhein_validator'
_description = 'rhein_validator.rhein_validator'
_inherit = 'stock.picking'

@api.multi
def button_validate(self):
loglevels.LOG_WARNING("=======MY METHOD RUNNING AS WELL========")
if self.picking_type_id.code == 'incoming':
if not self.env.user.has_group('purchase.group_purchase_user'):
raise ValidationError(_('You are not authorized to validate this picking'))
res = super(rhein_validator, self).button_validate()
return res

Hi,
In your class definition try as this by removing _name :
class rhein_validator(models.Model):
_inherit = 'stock.picking'
// Add function here

final code, that worked, hopefully it might help someone in same trouble. cheers.
==============================

# -*- coding: utf-8 -*-

# from xml.dom import ValidationErr
from odoo import models, fields, api
from odoo import _
from odoo.exceptions import ValidationError

class rhein_validator(models.Model):
_inherit = 'stock.picking'
def button_validate(self):
# logging.WARNING('THIS IS A WARNING MESSAGE'
print ("THIS IS A TEST MSG")
if self.picking_type_id.code == 'incoming':
if not self.env.user.has_group('purchase.group_purchase_user'):
raise ValidationError(_('You are not authorized to validate this picking'))
res = super(rhein_validator, self).button_validate()
return res