On Windows, IDLE closes all open windows and exits completely, without any error message, when selecting the "Print window" menu command.

Starting IDLE from inside a console, one can see the error message:

Exception in Tkinter callback
Traceback (most recent call last):
  File "D:\apps\python32\lib\tkinter\__init__.py", line 1399, in __call__
    return self.func(*args)
  File "D:\apps\python32\lib\idlelib\IOBinding.py", line 453, in print_window
    command = idleConf.GetOption('main','General','print-command-win')
  File "D:\apps\python32\lib\idlelib\configHandler.py", line 245, in GetOption
    type=type, raw=raw)
  File "D:\apps\python32\lib\idlelib\configHandler.py", line 54, in Get
    return self.get(section, option, raw=raw)
  File "D:\apps\python32\lib\configparser.py", line 789, in get
    d)
  File "D:\apps\python32\lib\configparser.py", line 391, in before_get
    self._interpolate_some(parser, option, L, value, section, defaults, 1)
  File "D:\apps\python32\lib\configparser.py", line 440, in _interpolate_some
    "found: %r" % (rest,))
configparser.InterpolationSyntaxError: '%' must be followed by '%' or '(', found
: '%s'

It is trying to read this entry from the config-main.def file:

[General]
print-command-posix=lpr %s
print-command-win=start /min notepad /p %s

For a ConfigParser file, those %s should be %%s instead. Previous IDLE versions (2.7 and 3.1) read the same entry without problem; I suspect they were using a RawConfigParser or processing the entry in a different way.

As a workaround, replacing %s with %%s in config-main.def is enough, until the code gets fixed.

Note: There is a much bigger problem here: IDLE should not abort abruptly in such cases, without any error indication.

New changeset 0aa3064d1cef by Łukasz Langa in branch '3.2':
bpo-12274: use proper escaping for % in IDLE config.
http://hg.python.org/cpython/rev/0aa3064d1cef

New changeset b410d013e7a8 by Łukasz Langa in branch 'default':
bpo-12274: use proper escaping for % in IDLE config (merged from 3.2).
http://hg.python.org/cpython/rev/b410d013e7a8

In 3.2 configparser started validating syntax in interpolation-aware parsers by default. I fixed the configuration.

It's unfortunate this configuration error hasn't been caught before in testing. We need a unit test for broken configuration and a change in IDLE so that it doesn't silently crash when configuration is invalid.

Can this be closed as a patch has been committed and the unittest framework was created on bpo-15392 ?

This issue needs a unittest test added within the framework. I opened bpo-21696 as a dependency. Since configx.def files can be edited, and, I believe, at least one is intended to be edited, configHandler should perhaps catch exceptions and display in a messagebox. But unless Idle could continue after that, that might wait for the general fix, which is another issue.

Closing since the immediate problem has been fixed and there is a reference to this in bpo-21696