AttributeError: 'MyReturnWindow' object has no attribute 'selected_index'

Question

So, I have got the following code in my second(return) window:

Why do I get the above error in returnMovie: index = self.selected_index.get()

I know that the returnMovie function isn't working, I am just curious why I get that error?

    def returnMovie(self):
        returningAccount = controll1.GetAccount(self.Input.get())
        return_dialog = MyReturnWindow(self, returningAccount)
        self.wait_window(return_dialog)

class MyReturnWindow(Toplevel):
    def __init__(self, parent, Account):
        super().__init__(parent)
        self.second_frame = Frame(self)
        self.second_frame.pack(fill=BOTH, expand=True)
        self.myReturnButton = Button(self, text='Return', command=self.returnMovie(Account))
        self.myReturnButton.pack(side=LEFT)

        self.selected_index = IntVar(self, -1)
        self.item_source = Variable(self, Account.get_movies())
        self.item_source.set(Account.get_movies())

        self.listbox_return = Listbox(self.second_frame, listvariable=self.item_source)
        self.listbox_return.bind("<<ListboxSelect>>", self.listbox_return_selection_changed)
        self.listbox_return.pack(fill=BOTH, expand=True)

    def listbox_return_selection_changed(self, event):
        index = self.listbox_return.curselection()[0]
        self.selected_index.set(index)
        self.selected_movie.set(controll1.GetMovies()[index])

    def returnMovie(self, Account):
        index = self.selected_index.get()
        movie = self.selected_movie.get()
        if index in range(0, self.listbox_return.size()):
            controll1.GetAccountMovies(Account).pop(index)
            controll1.GetMovies().pop(index)
            self.item_source.set(controll1.GetAccountMovies(Account))
        else:
            messagebox.showerror('No Movie found', 'There was no movie found')

Answer 1

You get the error because of this line of code:

self.myReturnButton = Button(self, text='Return', command=self.returnMovie(Account))

The above code is exactly the same as this:

result = self.returnMovie(Account)
self.myReturnButton = Button(self, text='Return', command=result)

Thus, you are calling self.returnMovie(Account) before setting self.selected_index, so it throws an error when it tries to reference that attribute.

When setting the command option you must provide a callable -- essentially the name of a function. Normally you should provide a command that doesn't require an argument, but since your command requires an argument then a simple solution is to use lambda:

self.myReturnButton = Button(..., command=lambda: self.returnMovie(Account))