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Making a modal window




Hello,
[I'M UNSURE WHETHER IT IS THE RIGHT PLACE TO ASK THIS QUESTION. IF NOT PLEASE GUIDE ME TO THE RIGHT PLACE]
I'm making a project(using Python3) which opens whenever someone opens their computer (assume Linux-Debain based) and asks them some details. The user should not be allowed to use the computer without giving the details. The project is based on GTK3 and is for Debian based OS. I need help in the following-
How can I make the application modal? That is, make sure that the user cannot access any thing in desktop without giving the details (the application is a compulsory one, thus one should not be able to close or minimise it).

I researched on this a bit and found the answers leading to Desktop Managers. But I'm stuck how to start with them. So need some help here.

Thank You
J. Arun Mani