What does “set the grab on something” mean?

Question

There's a function in the Python programming language, in particular in its tkinter module, that is called grab_set, which basically allows a Toplevel window to take complete control over the keyword and mouse events, so that these events are not sent to wrong windows. If you are interested, you can find more information in this article:

http://effbot.org/tkinterbook/tkinter-dialog-windows.htm

Now, someone told me that the correct expression related to this method was "setting the grab on a window", but it is not completely clear what it means, even if from the context I can imagine more or less its meaning.

See this article for context:

https://stackoverflow.com/questions/32259518/efficient-way-to-create-a-child-window-with-access-to-main-window-disabled/32259971#32259971

Is this a common expression? What exactly does it mean? In which contexts can I use it?

Examples are appreciated.

Answer 1

This is technical jargon and does not sound natural in English except in the very specific context it came from.

Window systems (GUIs) have the concept of "focus", i.e., the part of the screen that you are currently interacting with (where your keystrokes go). Sometimes people say that a particular window or UI element "has focus".

Usually focus is controlled by the user, but sometimes the program wants to direct the user's interaction to a particular thing. This is sometimes called "grabbing (the) focus", especially in X Window system environments (where Tkinter originated).

The "grab_set" methods sets a flag on the dialog window, requesting that it grab focus away from its parent window.

So, "setting the grab" here is a very shorthand way of saying "setting the flag/attribute that requests the window system to grab focus for this window when it is displayed".