[Tkinter-discuss] Proposal for thread-safe Tkinter

Allen Taylor Allen.Taylor at mdacorporation.com
Mon Oct 27 15:59:30 CET 2008

After a very helpful discussion with "Guilherme Polo <ggpolo at gmail.com>", we have come to the following conclusion regarding multithreaded applications and Tkinter. Although Tk is technically thread-safe (if built with --enable-threads), practically speaking it still has problems when used in multithreaded applications. The problem stems from the fact that _tkinter attempts to gain control of the main thread via a polling technique. If it succeeds, all is well. If it fails, however, the application receives an exception with the message "RuntimeError: main thread is not in main loop". There is no way to tell when this might happen, so calling Tk routines from multiple threads seems to be problematic at best.
The mtTkinter module I published last week (http://tkinter.unpythonic.net/wiki/mtTkinter) solves this problem by marshaling calls coming from multiple threads through a queue which is read by an 'after' event running periodically in the main loop. This is similar to the technique used in many other platforms (e.g., .NET's InvokeRequired/Invoke mechanism). The technique used in mtTkinter is exception-safe as well, marshaling exceptions through a response queue back to the caller's thread.
If a similar technique were employed in _tkinter instead of the polling technique, that would be a preferable solution. In the meantime, mtTkinter will work as a good stop-gap measure.
Allen B. Taylor
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