The code looks like that : 

 @api.one

    @api.depends('move_type','move_lines.state', 'move_lines.partially_available')

    def _state__get(self):

        result = super(stock_picking,self)._state_get(None,None)

        self.state = result[self.id]

        if self.state == 'assigned':

            self._update_intervention_tasks()

    state=fields.Selection(compute=_state__get, copy=False,

            store=True,

            selection=[

                ('draft', 'Draft'),

                ('cancel', 'Cancelled'),

                ('waiting', 'Waiting Another Operation'),

                ('confirmed', 'Waiting Availability'),

                ('partially_available', 'Partially Available'),

                ('assigned', 'Ready to Transfer'),

                ('done', 'Transferred'),

                ], string='Status', readonly=True, select=True, track_visibility='onchange',

            help="""

                * Draft: not confirmed yet and will not be scheduled until confirmed\n

                * Waiting Another Operation: waiting for another move to proceed before it becomes automatically available (e.g. in Make-To-Order flows)\n

                * Waiting Availability: still waiting for the availability of products\n

                * Partially Available: some products are available and reserved\n

                * Ready to Transfer: products reserved, simply waiting for confirmation.\n

                * Transferred: has been processed, can't be modified or cancelled anymore\n

                * Cancelled: has been cancelled, can't be confirmed anymore"""

                )