ON STATE CHANGE

# stock.py file

from openerp import models, api, fields


class MyExtension(models.Model):
    _inherit = 'stock.picking'

    some_instances = fields.One2many(comodel_name='some.some',
                                     inverse_name='return_picking')

    @api.onchange('state')
    def changed_return_picking_state(self):
        import ipdb; ipdb.set_trace()