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In response to my python-list request for testing tkinter/idle on linux, Petr wrote
On Fedora 21, the tests pass (Python 3.4.1 and latest dev version).
On Fedora 23, I see:
$ python3 -V
Python 3.4.3 <tests pass>
$ ./python -V
Python 3.6.0a0
$ ./python -m test -ugui test_tk test_ttk_guionly test_idle
[1/3] test_tk
[2/3] test_ttk_guionly
[3/3] test_idle
test test_idle failed -- Traceback (most recent call last):
 File "/home/pviktori/dev/cpython/Lib/idlelib/idle_test/test_configdialog.py",
line 27, in test_dialog
 d=ConfigDialog(self.root, 'Test', _utest=True)
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line
71, in __init__
 self.LoadConfigs()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line
1078, in LoadConfigs
 self.LoadFontCfg()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line
980, in LoadFontCfg
 self.SetFontSample()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line
858, in SetFontSample
 self.labelFontSample.config(font=newFont)
 File "/home/pviktori/dev/cpython/Lib/tkinter/__init__.py", line
1330, in configure
 return self._configure('configure', cnf, kw)
 File "/home/pviktori/dev/cpython/Lib/tkinter/__init__.py", line
1321, in _configure
 self.tk.call(_flatten((self._w, cmd)) + self._options(cnf))
_tkinter.TclError: expected integer but got ""
2 tests OK.
1 test failed:
 test_idle
Tk version: 8.6.4
---------------------------------------

I'd be happy to do any further testing (modulo a vacation until this Tuesday), but unfortunately I don't have time to dive into tkinter myself and look for the cause.
IDLE starts normally on this box.
Also note: Fedora 23 is currently in alpha (though Python should be stable in it).

Could you please add
 print(_flatten((self._w, cmd)) + self._options(cnf))
just before the call in line 858 in Lib/tkinter/__init__.py and tell the output Petr. Could you please show your .idlerc/config-main.cfg file?
I suppose self.fontSize.get() returns an empty string for unknown reasons. Interesting, how IDLE configuration file affects tests?

Config files affecting tests (and tests affecting config files!) is an issue. Would like to see config files written into a different directory when running tests. Any concerns with doing that?

Sorry for the delay; I lost the mail notification.
I don't have a config-main.cfg file.
The last output from the test, with the print added:
test_dialog (idlelib.idle_test.test_configdialog.ConfigDialogTest) ... ('.139655680391704', 'configure', '-borderwidth', 5)
('.139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676917048.139655676917672', 'configure', '-command', '139655677350312yview')
('.139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676917048.139655676917360', 'configure', '-yscrollcommand', '139655677341608set')
('.139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676917152.139655676918296', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676917152.139655676918296.menu>)
('.139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676917152.139655676918296', 'configure')
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932496.139655676932704', 'configure', '-state', 'disabled')
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932496.139655677333032.139655677332408', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680345608.139655676932496.139655677333032.139655677332408.menu>)
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932496.139655677333032.139655677332408', 'configure', '-highlightthickness', 0)
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935200', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935200.menu>)
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935200', 'configure')
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935616', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935616.menu>)
('.139655680391704.139655680370912.139655680371640.139655680345608.139655676932600.139655676935616', 'configure')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961480.139655676961688.139655676961896', 'configure', '-command', '139655676968648yview')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961480.139655676961688.139655676962000', 'configure', '-command', '139655676973368xview')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961480.139655676961688.139655676962104', 'configure', '-yscrollcommand', '139655676973928set')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961480.139655676961688.139655676962104', 'configure', '-xscrollcommand', '139655676974600set')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676963976', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676963976.menu>)
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676963976', 'configure')
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676964392', 'configure', '-menu', <tkinter.Menu object .139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676964392.menu>)
('.139655680391704.139655680370912.139655680371640.139655680789328.139655676961584.139655676963352.139655676964392', 'configure')
('.139655680391704.139655680370912.139655680371640.139655677346192.139655676990984.139655676992648.139655676992856', 'configure', '-command', '139655676998504yview')
('.139655680391704.139655680370912.139655680371640.139655677346192.139655676990984.139655676992648.139655676992960', 'configure', '-yscrollcommand', '139655677003496set')
('.139655680391704.139655680370912.139655680371640.139655680370184.139655676916736.139655676918920.139655676919024', 'configure', '-font', '{dejavu sans mono} {} normal')
ERROR
======================================================================
ERROR: test_dialog (idlelib.idle_test.test_configdialog.ConfigDialogTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/pviktori/dev/cpython/Lib/idlelib/idle_test/test_configdialog.py", line 27, in test_dialog
 d=ConfigDialog(self.root, 'Test', _utest=True)
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line 71, in __init__
 self.LoadConfigs()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line 1078, in LoadConfigs
 self.LoadFontCfg()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line 980, in LoadFontCfg
 self.SetFontSample()
 File "/home/pviktori/dev/cpython/Lib/idlelib/configDialog.py", line 858, in SetFontSample
 self.labelFontSample.config(font=newFont)
 File "/home/pviktori/dev/cpython/Lib/tkinter/__init__.py", line 1331, in configure
 return self._configure('configure', cnf, kw)
 File "/home/pviktori/dev/cpython/Lib/tkinter/__init__.py", line 1322, in _configure
 self.tk.call(_flatten((self._w, cmd)) + self._options(cnf))
_tkinter.TclError: expected integer but got ""
----------------------------------------------------------------------

This and #26673 are duplicates. The latter also reports failure on Arch Linux and other Python versions. While our default policy is to keep the earliest report open, I am closing this as #26673 has more information, including from Petr.