This is the code

_function_example(self, cr, uid, ids, name, arg, context=None):
    res = {}
    records = self.browse(cr, uid, ids)
    for record in records:
        res[record.id] = '%s%s' % (record.field_char_1, record.field_char_2)
    return res

_columns = {
    'your_function_field' : fields.function(_function_example, type='char', size=64, method=True, store=False),
}