[Tkinter-discuss] tkinter and threading

Jeff Epler jepler at unpythonic.net
Tue Jun 7 15:57:41 CEST 2005

No, Tkinter callbacks are not run in separate threads.

When you see cb_close being called during cb_run, it's all in a single
thread, with a call stack that looks like
i.e., when cb_run calls update, events (including "the user clicked a
button", "pressed a key", etc.) are handled, leading to this effect.

You may be able to use update_idletasks instead of update.  They have
different semantics as to which events they handle.  update_idletasks
should let things like resizing and repainting windows work, while
delaying the response to user actions like clicking until an update or a
return to the mainloop.

I've included a copy of the update manpage below.  Tcl's "update" command is
the .update() method, and "update idletasks" is the .update_idletasks() method.


update(n)                    Tcl Built-In Commands                   update(n)


       update - Process pending events and idle callbacks

       update ?idletasks?

       This  command is used to bring the application ``up to date'' by enter-
       ing the event loop repeatedly until all pending events (including  idle
       callbacks) have been processed.

       If  the  idletasks  keyword is specified as an argument to the command,
       then no new events or errors are processed;  only  idle  callbacks  are
       invoked.   This  causes  operations that are normally deferred, such as
       display updates and window layout calculations, to be performed immedi-

       The  update  idletasks  command is useful in scripts where changes have
       been made to the application's state and  you  want  those  changes  to
       appear  on  the display immediately, rather than waiting for the script
       to complete.  Most display updates are performed as idle callbacks,  so
       update idletasks will cause them to run.  However, there are some kinds
       of updates that only happen in response to events, such as those  trig-
       gered  by  window  size changes; these updates will not occur in update

       The update command with no options is useful in scripts where  you  are
       performing  a  long-running computation but you still want the applica-
       tion to respond to events such as user interactions;  if you  occasion-
       ally call update then user input will be processed during the next call
       to update.

       after(n), bgerror(n)

       event, flush, handler, idle, update

Tcl                                   7.5                            update(n)

On Mon, Jun 06, 2005 at 07:30:11PM -0700, Greg Lee wrote:
> Are Tkinter callbacks run in separate threads?  Do I need to use threading locks to protect data shared between callbacks that can run at the same time? I'm running Python 2.3 and Tkinter on Windows.
> I have two callbacks: cb_close and cb_run.  cb_close cleans up, then uses Tkinter.Toplevel.destroy; it is bound to WM_DELETE_WINDOW via Tkinter.Toplevel.protocol.
> cb_run uses a COM component to perform a lengthy operation.  The COM component calls back to the Python world often enough that I can keep my GUI responsive using Tkinter.Toplevel.update.
> Bad things happen if I allow cb_close to call destroy while cb_run is in progress: cb_run eventually asks the Tkinter world to post a completion dialog.  To keep this from happening I use a state variable to tell cb_close that it should not call destroy if cb_run is in progress and to tell cb_run that it should not execute if a cb_close is in progress.
> I can definitely get cb_close to run at the same time as cb_run, which suggests that the Tkinter eventloop executes its callbacks in separate threads.  All this lead me to use a lock to protect read/write of the state variable.  Was this necessary?  Is there something in the way that Tkinter handles callbacks that automatically protects shared data structures?
> --------------------------------------------
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