In a Model attribute order should follow the series given below.
Private attributes (_name, _description, _inherit, ...)
1. Default method and _default_get
2. Field declarations
3. Compute and search methods in the same order as field declaration
4. Constrains methods (@api.constrains) and onchange methods (@api.onchange)
5. CRUD methods (ORM overrides)
6. Action methods
7. And finally, other business methods.
For example you can see below code..

class Event(models.Model):
    # Private attributes
    _name = 'event.event'
    _description = 'Event'

    # Default methods
    def _default_name(self):
        ...

    # Fields declaration
    name = fields.Char(string='Name', default=_default_name)
    seats_reserved = fields.Integer(oldname='register_current', string='Reserved Seats',
        store=True, readonly=True, compute='_compute_seats')
    seats_available = fields.Integer(oldname='register_avail', string='Available Seats',
        store=True, readonly=True, compute='_compute_seats')
    price = fields.Integer(string='Price')

    # compute and search fields, in the same order that fields declaration
    @api.multi
    @api.depends('seats_max', 'registration_ids.state', 'registration_ids.nb_register')
    def _compute_seats(self):
        ...

    # Constraints and onchanges
    @api.constrains('seats_max', 'seats_available')
    def _check_seats_limit(self):
        ...

    @api.onchange('date_begin')
    def _onchange_date_begin(self):
        ...

    # CRUD methods
    def create(self):
        ...

    # Action methods
    @api.multi
    def action_validate(self):
        self.ensure_one()
        ...

    # Business methods
    def mail_user_confirm(self):
        ...

Note-Private attributes (_name, _description, _inherit, ) is used to declare model.