The proposed patch makes the font sample in IDLE font configuration dialog editable. This allows users to test fonts with arbitrary samples.

Thanks for the patch.  Adding the feature is somehow easier than I expected.  After moving the sample text to module level, which I considered doing before, saving edits for the duration of an IDLE session turned out to also be easy.  

With 11 point Lucida Console, there is room for 5 more lines, without erasing anything, before anything scrolls off the top.

I expect that saving changes across IDLE sessions would be much harder and likely not worth the effort.  I think that exploring font choices is likely rare enough that there is little need to do so.

I don't know if saving the changes would be too difficult.

1.  Create a new config file for the text (I think it would clutter the existing config files, but could also add it there).
2.  Load font_sample_text from the config file.
3.  In apply(), write the text back to the config file.

Issues:
1.  Have a reset button so that the original sample text could be restored at some point?  If so, then need to store the original (current) text somewherem maybe a .def and .cfg version like for the other settings?  A reset button would also require some screen space from the text frame.

Cheryl, thanks for explaining, in part, why I don't want to do this ;-).  Using the the current .def, .cfg system would mean that the default sample would be frozen.  I want to be able to change it.

Makes sense.  I guess I didn't think of it being frozen because I was thinking of it being more like the Recent Files list, but pre-populated if it were empty.  As you said, probably not worth the effort.  :-)

New changeset ed6554c487fb2403bc88be6deee611c7a4171d33 by Terry Jan Reedy (Serhiy Storchaka) in branch 'master':
bpo-31860: Make the font sample in the IDLE font configuration dialog editable. (#4106)
https://github.com/python/cpython/commit/ed6554c487fb2403bc88be6deee611c7a4171d33

New changeset 6a2957de08e0c2d73f3124d12874b408cda4633d by Terry Jan Reedy (Miss Islington (bot)) in branch '3.6':
bpo-31860: Make the font sample in the IDLE font configuration dialog editable. (GH-4106) (#4154)
https://github.com/python/cpython/commit/6a2957de08e0c2d73f3124d12874b408cda4633d

Postscript: I read a week or so ago that a new Windows 10 font chooser will have this feature.  There was not much detail and I believe it is currently only part of the Preview package.