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While working on bpo-31068, I wrote attached widget_instance.patch to check if test_ttk_guionly really removes all widgets. Bad news: it doesn't. Example:
haypo@selma$ ./python -m test -u all test_ttk_guionly
(...)
test test_ttk_guionly crashed -- <type 'exceptions.Exception'>: leaking 69 widgets
(...)
Attached fix_destroy.patch is my attempt to fix this issue.

I only tested Python 2.7. I didn't check if other branches are affected.

See issue25130. Maybe forcing garbage collecting will help.

My widget_instance.patch calls test_support.gc_collect() and removes all
widgets from the root window. It's not enough. That's why I proposed to modify LabeledScale.destroy().

On master, LabeledScaleTest.test_initialization() leaks a widget in the root widget, even after root.destroy(). The problem is that LabeledScaleTest.__del__() doesn't call Frame.destroy(self) if self._variable is not set. So master has at least this bug.

widget_instance-master.patch: Check that Tkinter.Widget.destroy() removes all children and check that test_ttk_guionly doesn't leak widgets.

Example of failure on master using widget_instance-master.patch:
haypo@selma$ ./python -m test -v -u all test_ttk_guionly 
======================================================================
ERROR: test_initialization (tkinter.test.test_ttk.test_extensions.LabeledScaleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/haypo/prog/python/master/Lib/tkinter/test/test_ttk/test_extensions.py", line 14, in tearDown
 super().tearDown()
 File "/home/haypo/prog/python/master/Lib/tkinter/test/support.py", line 40, in tearDown
 w.destroy()
 File "/home/haypo/prog/python/master/Lib/tkinter/__init__.py", line 2305, in destroy
 % self.children)
Exception: destroy() doesn't clear all children: {'!labeledscale2': <tkinter.ttk.LabeledScale object .!frame.!labeledscale2>}
My https://github.com/python/cpython/pull/3025 PR fixes LabeledScale and OptionMenu destroy() method in the master branch.

widget_instance-2.7-v2.patch: Updated patch for Python 2.7 to debug test_ttk_guionly.
https://github.com/python/cpython/pull/3026 fix LabeledScale and OptionMenu destroy() method for Python 2.7.

New changeset 56e162ad5c5d3effe9b4f05d0179e1b6a2a2d9b8 by Victor Stinner in branch '2.7':
ttk: fix LabeledScale and OptionMenu destroy() method (#3026)
https://github.com/python/cpython/commit/56e162ad5c5d3effe9b4f05d0179e1b6a2a2d9b8

New changeset cd7e9c1b67d3d07ee03e0a79af2791c19031cecb by Victor Stinner in branch 'master':
ttk: fix LabeledScale and OptionMenu destroy() method (#3025)
https://github.com/python/cpython/commit/cd7e9c1b67d3d07ee03e0a79af2791c19031cecb

New changeset 33460fa7e0bd126bee739a66e1228665dc22e70f by Victor Stinner in branch '3.6':
ttk: fix LabeledScale and OptionMenu destroy() method (#3025) (#3030)
https://github.com/python/cpython/commit/33460fa7e0bd126bee739a66e1228665dc22e70f

Copy of interesting comments: https://github.com/python/cpython/pull/3025
serhiy-storchaka: "Parent's destroy() now is called even if this destroy() already was called. I.e. it can be called twice."
haypo: "Yes, it's a deliberate choice. All other ttk widgets now have the same behaviour."
--
Ok, the bug is now fixed in 2.7, 3.6 and 3.7 (master) branches. I close the issue.