Attached is a unittest for idlelib.Percolator.
2.7 version will be added once this is OK.

I've reviewed the patch and made my remarks in the review tool.

These tests don't test the central functionality of Percolator nearly enough. We should test, at least:

1) That the text actually went through the filter (and not directly to the Text
widget).
2) That if a filter modifies the text, the modified text appears in the Text
widget.
3) That a filter can stop the event from continuing through the following
filters.
4) That having more than one filter modify the arguments works.
5) That not having any filters works.
6) That this doesn't only work on a Text widget's "insert" and "delete" events.

There are probably more things we should test. Take a look at what this is used
for in practice in the code for ideas.

New changeset eebccf05903e by Terry Jan Reedy in branch '3.5':
Issue #21939: Test IDLE percolator.  Original patch by Saimadhav Heblikar.
https://hg.python.org/cpython/rev/eebccf05903e

Tal, thanks for the suggestions.  Saimadhav added tests for at least some of them.  Part of the difficulty in adding IDLE tests is that it is too easy to write tests to the code instead of the use of the code.  This is especially true when side-effects (changing a widget) are the main intended effect.  I plan to work more on the latter.