Dear Abdelwahed,

There are lots of examples of onchange in addons.

Here I given simple example of onchange method

Definition of on change methods in a view looks like this:

<field name="name" on_change="name_change(name, address, city)"/>

And here is the corresponding method in the model:

def name_change(self, cr, uid, ids, name, address, city, context=None):

...

    return {

        'value': {

            'address': ...

            'city': ...

        }

}

2nd Example

In xml file or view file.

<field name="partner_id" on_change="onchange_partner(partner_id)"/>

In python file or model file

def onchange_partner(self, cr, uid, ids, partner_id, context=None):

res = {}

if partner_id:

    obj = self.pool.get('res.partner').browse(cr, uid, partner_id, context=context)

    res['field1'] = obj.field1

    res['field2'] = obj.field2

    res['field3'] = 'Hello' # this field type must be char or if not then give

    #value accordingly, its just to prove you that values are filled on onchange.

    return {'value': res}

Hope this will help you. 

Thanks & Regards

Ankit H Gandhi

inherit class a in b, then change your field value.