Hi, have you solved this problem? may i know how you can solved this problem because right now i got same problem like you. Thank you

How do you display the wizard form ??
Are you save the wizard record first then show the record at pop up wizard  OR you just set new form with some default value ??

Example:

1.  https://pastebin.com/ymzMcQFF

2. \https://pastebin.com/ymzMcQFF
   

Based on your syntax (def action_plus(self):),, self pointed to the object (including record) that to be executed.. so if you use style no #2 you can't do that (increase/decrease) only just by the python,, but if you use style no #1 it could be done because when you call the button the record has been saved at database and odoo know wich record to be updated..
You can test you code with "print(self)", or logger to the command line,, and see what the result. I guess the result will be a empty object...
Ex:

@api.multi

def action_plus(self):

   # DO THIS FOR SEE "SELF" VALUE--->  print(self)
   # OR DO THIS --> _logger.info(self) # IF YOU USE logger object

    for record in self:

        record.qty  = record.qty + 1

But i think its better you to handle it by javascript,,  then user can save it with save button..

Regards

La Jayuhni Yarsyah

Hi, it seems it is because your page is not refreshed after the action is done. If you reload the page, it should be updated. 

To programatically refresh the page, try something like:

return {
    'type': 'ir.actions.client',
    'tag': 'reload',
}

You can also play around with 'always_reload' and 'reload_on_button' options for the QTY field. Have a look at: https://www.odoo.com/forum/ayuda-1/question/what-behavior-with-options-always-reload-5566

Finally, I guess it was a misprint, but anyway: the decorator is @api.multi. In your question you missed 'i' at the end