check whether a customer name already exists

_openerp_.py contains general data as in other modules.And the .py file contains the above updated code.And then the module was added to the addons folder and the openerp was updated .When it was listed in the Installed modules list,it was installed to the modules and once again the modules list was updated so that it can be used.After doing all these procedures when i tested by entering the customer name that already existed,the expected validation didnt work