PEP-8 specifies a limit of 72 chars for flowing text (comments, multiline strings). The current default limit for Idle's Format / Reformat Paragraph is 70. Increase it to PEP-8's 72.

Changing the default in the default configuration file is trivial. In test_formatparagraph.FormatEventTest, test_comment_block and test_long_line currently fail if the reformat width is changed on the General tab of the configuration dialog. These tests will need to be altered when the default is changed. Also, the tests should temporarily change the reformat width to the presumed default in the class setup and teardown methods. If this is not possible, the tests should be skipped if it is not what the test requires.

Attaching a patch for 2.7 and 3.4
The comment strings are modified to reflect 70->72 in the tests.

I am puzzled at the differences between the 2.7 and 3.4 patches. There are only three differences between the files

- from Tkinter import Tk, Text, TclError
+ from tkinter import Tk, Text, TclError
?      ^
- from test.test_support import requires
+ from test.support import requires
?           ^
-     get_selection_indices = EditorWindow.get_selection_indices.im_func
+     get_selection_indices = EditorWindow. get_selection_indices
 
and none of these should affect the patch.

The attached patch adds a limit parameter to the FormatParagraph.format_paragraph_event method ReformatParagraph.py that is called as self.formatter in the tests.
cls.formatter = fp.FormatParagraph(editor).format_paragraph_event
With this change, we can augment the calls to
self.formatter('ParameterDoesNothing', 72)
and the tests will ignore the configured value.

I applied my patch as part of bpo-21284. When I did so, I added 'limit=70' so that the tests pass otherwise unchanged. The only thing left here is to change config-main.def.

I put 21138 in the commit message. The push messages are these.
New changeset 374746c5dedc by Terry Jan Reedy in branch '2.7':
Issue bpo-21138: Change default reformat paragraph width to PEP-8's 72.
http://hg.python.org/cpython/rev/374746c5dedc

New changeset dd24099c0cf6 by Terry Jan Reedy in branch '3.4':
Issue bpo-21138: Change default reformat paragraph width to PEP-8's 72.
http://hg.python.org/cpython/rev/dd24099c0cf6