I have C code which handles I/O and needs an event
dispatcher. It is running in the context of a Python
GUI application, and uses the Tcl event loop, using
Tcl_CreateFileHandler and friends. The C code gets
callbacks from the Tcl event loop fine, but when it
attempts to call into the Python application from one
of these callbacks, things go wrong.

• Tkinter has released the GIL and hidden the current
  thread state. I can work around this by re-acquiring
  the GIL from the callback and creating a new thread state.

• When the callback is invoked, Tkinter's tcl_lock is
  held. If the Python code invoked from the callback
  ultimately calls some other Tkinter function, the
  tcl_lock is still held, and deadlock results. The only
  way to work around this is to use a single-threaded
  Python build.

• If the Python code returns an error, there's no way
  to stop the event loop to report the error up.
  Tkinter's error-reporting mechanisms are inaccessible.

In general, Tkinter has a lot of infrastructure for
managing callbacks from the Tcl event loop. If a third
party C library wants to use the same event loop, that
infrastructure is unavailable, and it is very difficult
to work with Python.

Unfortunately, short of using threads (which have their
own problems), there's no other alternative for an
external C library to do I/O without blocking the GUI.
I've seen several problem reports from people trying
to do exactly this, though they almost never figure out
all of what's going on, and nobody else ever has any
good advice to offer.

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For lack of a volunteer or better victim... er, assigning to /F

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Unassigning -- /F is a black hole. :-(

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It seems that all you need to have is access to the Tcl
lock. Would that solve your problem?

As for reporting the error up: This is certainly possible.
Just implement a _report_exception method on your widget;
define it as

    def _report_exception(self):
        raise

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I *think* external access to the tcl_lock would do it (it's
been a while). There are a bunch of helper functions/macros
inside the Tkinter code for handling this situation;
exposing and documenting those would be ideal.

As far as error reporting, I don't think that suffices.
(Again, it's been a while.) The problem is that the
exception is returned to the C side, which must figure out
what to do with it. Specifically, the Tcl event loop should
be stopped and the exception reported to whoever invoked
it... but there's no direct way to stop the Tcl event loop.
(Fuzzy memories here.)

The bug report is several years old and probably outdated. Please create
a new issue if you are still interested in the problem.