Hi,

You could use the "read" and "write" attributes on field definition, like this one:

    _columns = {
        'credit_limit': fields.float(string='Credit Limit',
                                     read=['base.group_sale_salesman'],
                                     write=['base.group_sale_manager']),

This will create a field named credit_limit that will be readable by users belonging to base.group_sale_salesman and writable only by base.group_sale_manager users...