Passing data between windows in Tkinter using object-oriented approach - python-3.x

Question

Working with tkinter, I have a main window that reads from a TSV file and creates a list of the rows and their values, and creates buttons based on each of the items. When you click a button, it opens a secondary TopLevel to edit the item you clicked. When completed, I want to pass the saved data back to the main window so it can be saved back to the list which can then be fully written back to the TSV.
I've searched and perused several SO and blog posts on this topic, the closest one being this:
How to pass data between top levels in tkinter
However, in this solution creates the widgets for the new (edit) window in the same class as the root and I'd like to separate the work for this into its own class. The problem is, when I pass the saved value back to the main window, I get an error (see below after code).
Here is a simplified version of the code I'm using (note, each of these classes will be in their own separate file, but for the sake of this demo, they are combined.
#!/usr/bin/python3
from tkinter import *
from tkinter import ttk
import os
from tkinter import messagebox
class MainApp:
def __init__(self, master):
self.testVar = "none"
self.master = master
ttk.Button(self.master, text = "Open Window", command=lambda: NewWindow(self.master)).pack()
class NewWindow(Toplevel):
def __init__(self, master = None):
super().__init__(master = master)
self.title("New Window")
self.geometry("200x200")
label = Label(self, text ="This is a new Window")
label.pack()
ttk.Button(self, text = "Save", command=lambda: self.SaveData()).pack()
def SaveData(self):
messagebox.showinfo(title=None, message="Save complete: " + self.master.testVar)
def main():
os.system('cls') # on windows
root = Tk()
MainApp(root)
root.mainloop()
if __name__ == "__main__": main()
Here is the error:
Exception in Tkinter callback
Traceback (most recent call last):
File "C:\Python39\lib\tkinter\__init__.py", line 1885, in __call__
return self.func(*args)
File "C:\repos\legend-bowl\test.py", line 22, in <lambda>
ttk.Button(self, text = "Save", command=lambda: self.SaveData()).pack()
File "C:\repos\legend-bowl\test.py", line 25, in SaveData
messagebox.showinfo(title=None, message="Save complete: " + self.master.testVar)
File "C:\Python39\lib\tkinter\__init__.py", line 2347, in __getattr__
return getattr(self.tk, attr)
AttributeError: '_tkinter.tkapp' object has no attribute 'testVar'
I would think that the instance of NewWindow should be able to access values off of its parent since the instance of the parent is passed into its constructor. I feel like I'm missing something simple that the other solutions haven't been able to answer for me.
Should I be able to access from the root on its children?
Obviously, there might be better ways to handle this that I'm not thinking of, so if you offer that as a solution, I'd still like to know the answer to #1

Answer 1

The problem is that self.master doesn't refer to the instance of MainApp, it refers to the root window. This is because you don't pass in the instance of MainApp to NewWindow, and self.master refers to a variable defined by tkinter.
You need to pass the instance of MainApp to NewWindow, save it, and then refer to it when trying to access testVar.
One way would be to do something like this (though personally I think a function would be better than lambda here):
ttk.Button(self.master, text = "Open Window", command=lambda: NewWindow(self, self.master)).pack()
Next, you have to define NewWindow to accept and save the parameter, and use it instead of self.master
class NewWindow(Toplevel):
def __init__(self, mainapp, master = None):
super().__init__(master = master)
self.mainapp = mainapp
...
def SaveData(self):
messagebox.showinfo(title=None, message="Save complete: " + self.mainapp.testVar)