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tkinter runs a busy-wait loop on interactive Python #140488
Description
Bug report
Bug description:
In EventHook in Modules/_tkinter.c, we used to have the following block to run the event loop:
#if defined(WITH_THREAD) || defined(MS_WINDOWS)
Py_BEGIN_ALLOW_THREADS
if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
tcl_tstate = event_tstate;
result = Tcl_DoOneEvent(TCL_DONT_WAIT);
tcl_tstate = NULL;
if(tcl_lock)PyThread_release_lock(tcl_lock);
if (result == 0)
Sleep(Tkinter_busywaitinterval);
Py_END_ALLOW_THREADS
#else
result = Tcl_DoOneEvent(0);
#endifin bpo-31370, support for threads-less builds was removed, and this block became
Py_BEGIN_ALLOW_THREADS
if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
tcl_tstate = event_tstate;
result = Tcl_DoOneEvent(TCL_DONT_WAIT);
tcl_tstate = NULL;
if(tcl_lock)PyThread_release_lock(tcl_lock);
if (result == 0)
Sleep(Tkinter_busywaitinterval);
Py_END_ALLOW_THREADSThis is a bit unfortunate, as we now back to running a busy-wait loop. See Guido van Rossum's comment of May 23, 1998: 7bf15648
I guess this block could be
if (self->threaded) {
/* Allow other Python threads to run. */
ENTER_TCL
result = Tcl_DoOneEvent(0);
LEAVE_TCL
}
else {
Py_BEGIN_ALLOW_THREADS
if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
tcl_tstate = tstate;
result = Tcl_DoOneEvent(TCL_DONT_WAIT);
tcl_tstate = NULL;
if(tcl_lock)PyThread_release_lock(tcl_lock);
if (result == 0)
Sleep(Tkinter_busywaitinterval);
Py_END_ALLOW_THREADS
}in analogy with the event loop in _tkinter_tkapp_mainloop_impl. As self->threaded is true in most builds, this would avoid the busy-wait loop.
I am happy to submit a PR, but I'll wait until PR #140147 is in to avoid unnecessary complications to this code.
CPython versions tested on:
CPython main branch
Operating systems tested on:
macOS
Activity
serhiy-storchaka commented on Jun 29, 2026
@mdehoon, #148819 is merged. Note EventHook must stay polling on Windows since stdin isn't a Tcl file handler there, unlike mainloop.
mdehoon commented on Jul 1, 2026
OK I can try and submit a PR for this issue.
Yes, you're right.
serhiy-storchaka commented on Jul 15, 2026
@mdehoon, gentle ping.
mdehoon commented on Jul 16, 2026
@serhiy-storchaka
Could you have a look at PR #152841 first, which is a trivial patch removing some dead code?
The reason is for the current issue, we will need to move threaded out of TkappObject and make it a single static variable, as in EventHook we won't have a TkappObject *self argument like in _tkinter_tkapp_mainloop_impl.
Logically it also makes sense for threaded to be a single static variable, as it is tied to the installed Tcl/Tk being threaded or not, as opposed to a particular TkappObject.
The dead code in PR #152841 also refers to the threaded variable, so I'd like to move it out of the way first.
serhiy-storchaka commented on Jul 17, 2026
I prefer first to apply backportable fixes, and only after that drop support of non-threaded Tcl in main.