Call a varaible from class in the parent class

[Clement_2000]

Hi experts.
I'm making a modular interface containing tabs with different tools.
These tools are for now :

• A qr code display for my math teacher.
  
• A real-time schedule display of my city's public transport company (the "CTCUB", aka "Bibus")
  
• And a calculator.
  

And the upcoming tools are :

• A text editor
  
• A graphical editor and calculator for math
  
• An easy formula editor and calculator for math and physics
  
• A print manager for my two beautiful dot matrix printers.
  
• Some other tools that I'll need in the future.
  

The principle, is that I have a main file, in which I import all my tools like an add-on. Then, by running a class, it automatically creates a tab and adds all the widgets in it.
And I'm having trouble making a calculator because I end up creating a class for each button (which seems to be the best way to do it). The problem, is that I end up having a class, into a class . But I don't know how to access a class variable from a child class.

Here is the zip containing all the files so you can see.
http://clement2000.rf.gd/doc/tmp/utilitaireTravail.zip
I added a little comment (line 12, in the init of the calculator folder) to show exactly where I'm having trouble.
Sorry, the GUI is in french, but for international purposes, all the comments ans code is in English. If translation needed, please ask.

I hope you did understand what I want, it's really complex.

Thanks in advance for your help,
Clément.