Hello 

i use odoo version 8 and i use new API in my code..
this is my code: 

test.py

from openerp import models, fields, api

class purchase_order(models.Model):

_inherit = 'purchase.order

name_name = fields.Char(string="Test")

def button_test(self):

for rec in self:

self.name_name = '123'

test_view.xml

<?xml version="1.0"?>

<openerp>

<data>

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

        <field name="name">test_purchase</field>

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

        <field name="inherit_id" ref="purchase.purchase_order_form"/>

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

<xpath expr="//header/button[@name='action_cancel']" position="after">

                <button name="button_test" string="TEST" type="object"/>                    

            </xpath>

        </field>

    </record>

</data>

</openerp>

but i always have this error when i click the button 

Traceback (most recent call last):
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 546, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 583, in dispatch
    result = self._call_function(**self.params)
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 319, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "D:\odoo8\odoo8\odoo8\openerp\service\model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 316, in checked_call
    return self.endpoint(*a, **kw)
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 812, in __call__
    return self.method(*args, **kw)
  File "D:\odoo8\odoo8\odoo8\openerp\http.py", line 412, in response_wrap
    response = f(*args, **kw)
  File "D:\odoo8\odoo8\odoo8\openerp\addons\web\controllers\main.py", line 948, in call_button
    action = self._call_kw(model, method, args, {})
  File "D:\odoo8\odoo8\odoo8\openerp\addons\web\controllers\main.py", line 936, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
TypeError: button_test() takes exactly 1 argument (5 given)