I propose that IDLE only allows a single instance, but behaves mostly like before (multiple shells, etc.). The main motivation for this issue is to (1) stop the same file being opened more than once and (2) make a tabbed interface easier to implement.

Starting with point (1), believe a file should not be allowed to be opened multiple times but enforcing this currently in IDLE would be incredibly difficult.

Instead, I propose that a socket-server sits on the main instance and any new instances send requests to the main instance (to open a file in a new 'instance'). There would be two layers of file-lists, and the current one still acts as an 'instance' file-list but we also create a master list which controls all of the 'instances':

Instance flist      Instance flist
     |                   |
     ---- Master flist ----

Secondly, point (2). I am currently in the planning phase of creating an IDLE tabbed interface (based loosely off the code currently found in #9262) but it requires both this issue and #40893 to be pulled before it can work effectively (dragging tabs between windows, etc.).

I don't currently have any code to propose, but I don't think it should be *too* difficult to implement (but now I've said that!...).