Description

mdehoon

opened on Oct 23, 2025

Issue body actions

Bug report

Bug description:

Modules/_tkinter.c always defines TCL_THREADS:

/* If Tcl is compiled for threads, we must also define TCL_THREAD. We define
   it always; if Tcl is not threaded, the thread functions in
   Tcl are empty.  */ 
#define TCL_THREADS

but in two places in Modules/_tkinter.c , we check if TCL_THREADS is defined:

#ifdef TCL_THREADS
static Tcl_ThreadDataKey state_key;
typedef PyThreadState *ThreadSpecificData;
#define tcl_tstate \
    (*(PyThreadState**)Tcl_GetThreadData(&state_key, sizeof(PyThreadState*)))
#else
static PyThreadState *tcl_tstate = NULL;
#endif

and

#ifndef TCL_THREADS
    if (v->threaded) {
        PyErr_SetString(PyExc_RuntimeError,
                        "Tcl is threaded but _tkinter is not");
        Py_DECREF(v);
        return 0;
    }
#endif

Especially the latter is confusing, as this code block is not compiled and the test is never executed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-140494: require that Tcl is built with thread support #141241
• gh-140494: Remove dead code ifndef TCL_THREADS, which is always defined #152841

Activity

1. mdehoon

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 23, 2025
2. terryjreedy

   added

   topic-tkinter

   on Oct 23, 2025
3. terryjreedy commented on Oct 23, 2025

   

   terryjreedy

   on Oct 23, 2025

   Member

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   tcl/tk 8.5 was by default compiled without thread_support. 8.6 by default is, and perhaps the option is gone. Thread support was once optional in python, but now is mandatory so _tkinter should support them also.

   I suspect that the define and the comment before could be eliminated (if these are the only two uses) and the code between #ifdef TCL_THREADS and #ELSE# made unconditional and the other code eliminated. @serhiy-storchaka ?
4. StanFromIreland

   added

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   type-refactorCode refactoring (with no changes in behavior)Code refactoring (with no changes in behavior)

   and removed

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 23, 2025
5. serhiy-storchaka commented on Oct 23, 2025

   

   serhiy-storchaka

   on Oct 23, 2025

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   It was unconditionally enabled in b5bfb9f, 23 years ago. Well, I think we can remove this code now. Note that some comments may be need to be updated.

   Reacted by yihong, A.Ibrahim and mdehoon
6. mdehoon commented on Oct 24, 2025

   

   mdehoon

   on Oct 24, 2025

   ContributorAuthor

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   We still need to #define TCL_THREADS , otherwise tkinter won't compile.

   Should we require that tcl/tk was compiled with thread support, or should we still allow tcl/tk without thread support?
7. serhiy-storchaka

   added

   3.16new features, bugs and security fixesnew features, bugs and security fixes

   on Jul 17, 2026
8. serhiy-storchaka commented on Jul 17, 2026

   

   serhiy-storchaka

   on Jul 17, 2026

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   I think we can drop support of non-threaded Tcl/Tk in main. AFAIK, 8.6 is threaded by default, and 9.0 is always threaded. But the minimal supported version was still 8.5.12. Some distributors can still need to build without threaded support, they may even patch #define TCL_THREADS. So this is main-only change.

   To make backporting easier, it is better to hold these changes until we fix other issues.

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