Hello Apoorv Soral,

If you want to override res.users method then You need to used super method for this.

For Ex:

class res_users(osv.osv):
    _inherit = "res.users"

     def check_credentials(self, cr, uid, password):
         res = super(res_users, self).check_credentials(cr, uid, password)  //For Override existing method
         print "res ========= ", res
         user_id = self.search(cr, SUPERUSER_ID, [('id','=',uid),('password','=',password)])
         print "user_id !!!!!!! ", user_id

         if user_id:

             return user_id

         else:

             return res

Hope this will helps you.

Thanks,