I have tried many combinationsfor _order and _parent_order of account_account class. Always get the same error. 

For example, If I tried to change _order directly in the original account.py file like this :

class account_account(osv.osv):
     _order = "code asc" ###instead of _order = "parent_left"    
     _parent_order = "code"
     _name = "account.account"
     _description = "Account"
     _parent_store = True

For all combinations I have tried, I got this error :

  File "/home/odoo-iv/odoo-8.0-20170119/openerp/addons/account/account.py", line 364, in __compute
    sums[current.id][fn] += sums[child.id][fn]
KeyError: 144

So, I have decided to change 'parent_left' and 'parent_right' field values for accounts in account.account database's table. I did it manually directly in the database.

Now I have this :

Thanks for your answers

dear Pascal