Tkinter: Why does it bring an empty variable that it gets from a class?

Question

I've been looking for a solution to my problem too long. I've reviewed all the pages that are proposed in the following question, exhaustively: Switch between two frames in tkinter, I have documented myself reviewing information from classes on the official python page, I've been reading about self, in case I'm not considering something; I have also gone through more pages, questions and solutions that I have come across along the way and rearranged my code several times, but I cannot find the solution and need help.

The code, as simplified as possible, is the following:

import os, easygui
import xml.etree.ElementTree as ET
import tkinter as tk

LARGE_FONT = ("Verdana", 12)


class PageOne(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)
        self.controller = controller

        print("Second TC:", self.controller.shared_data['tc'])

        for i in self.controller.shared_data['tc']:
            print("****: ", i)
            print("Name:", i['Name'], "Id:", i['Id'])


class StartPage(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)
        self.controller = controller

        label = tk.Label(self, text='File Upload: Press the button to open the XML file.', font=LARGE_FONT)
        label.place(relx=.01, rely=.01)

        # Function to separate the tc "Child_4" inside the xml file
        def separate_tc(xml_name):
            file_xml = ET.parse(xml_name)
            # Get the name and id attributes that the Child_4 tag contains
            self.controller.shared_data['tc'] = [
                {"Name": signal.attrib["Name"],
                 "Id": signal.attrib["Id"],
                 } for signal in file_xml.findall(".//Child_4")
            ]
            print('First TC:', self.controller.shared_data['tc'])
            controller.show_frame("PageOne")

        # Function to open xml file
        def open_file():
            try:
                xml_name = str(easygui.fileopenbox(title='Select XML file', default='*.xml'))
                if str(os.path.abspath(xml_name)) != os.path.join(os.path.abspath(os.getcwd()),
                                                                   os.path.basename(xml_name)):
                    separate_tc(os.path.basename(str(xml_name)))
                else:
                    separate_tc(os.path.basename(str(xml_name)))
            except FileNotFoundError:
                print('XML file was not loaded.')

        button_open = tk.Button(self, text="Open File XML", command=open_file)
        button_open.place(relx=.01, rely=.05)


class AppMain(tk.Tk):
    def __init__(self, *args, **kwargs):
        tk.Tk.__init__(self, *args, **kwargs)
        tk.Tk.wm_title(self, 'Analysis of a XML')

        self.shared_data = {'tc': []}

        container = tk.Frame(self)
        container.pack(side='top', fill='both', expand=True)
        container.grid_rowconfigure(0, weight=1)
        container.grid_columnconfigure(0, weight=1)

        self.frames = {}

        for F in (StartPage, PageOne):
            page_name = F.__name__
            frame = F(parent=container, controller=self)
            self.frames[page_name] = frame
            frame.grid(row=0, column=0, sticky="nsew")

        self.show_frame("StartPage")

    def show_frame(self, page_name):
        '''Show a frame for the given page name'''
        frame = self.frames[page_name]
        frame.tkraise()


if __name__ == "__main__":
    app = AppMain(None)
    app.title('Analysis of XML')
    app.geometry("1024x920")
    app.mainloop()

What I want to do is obtain the value of tc_xml and that is why I am assigning it in the variable controller.shared_data ['tc'] in the Home class, as I think I understand from the response of Storing data in the controller, but at the time of sending calling the variable in the PageOne class, brings it to me empty and I don't understand why. If someone could help me I would be very grateful, please.

Answer 1

In the comments you say you want this bit of shared data to be a StringVar, but then you set it to a list of dictionaries. I don't understand why you need to use a StringVar. Storing a list of dictionaries in a StringVar won't still be a list of dictionaries when you take the data back out of the StringVar.

I think you're overcomplicating the problem, Within the app you can use self.shared_data['tc'], and within any other class that has a self.controller property you use self.controller.shared_data['tc'].

For example, in the Home class, inside the separate_tc function, you can set the value like you would any other dictionary:

class Home(tk.Frame):
    def __init__(self, parent, controller):
        super().__init__(parent)
        self.controller = controller
        ...
        def separate_tc(xml_name):
            file_xml = ET.parse(xml_name)
            self.controller.shared_data['tc'] = [
                {"Name": signal.attrib["Name"],
                "Id": signal.attrib["Id"],
                } for signal in file_xml.findall(".//Child_4")
            ]

Later, you can use this data in the PageOne class:

class PageOne(tk.Frame):
    def __init__(self, parent, controller):
        super().__init__(parent)
        self.controller = controller
        ...
        for item in self.controller.shared_data['tc']:
            print("name:", item['Name'], "id:", item['id'])

The only remaining piece of the puzzle is that you need to define the shared data before you attempt to use it. In the code you shared, self.shared_data['tc'] isn't defined until you try to open a file. However, both PageOne and PageTwo try to access the data when the classes are first created rather when they are first shown. You must make sure that you don't try to use the data before you define it.