I am dubious that the current code will work with tkinter. Marks name slice positions. Tags name 0 to many pairs of slice positions, with 'sel' predefined as corresponding to a mouse selection and limited to one pair. According to my tests, tag_add('sel.first', None) adds a *new* tag that has nothing to do with the 'sel' selection tag. Tag.first and tag.last, if not used themselves as tag names, refer to the first and second slice positions of the first pair. Idle only uses 'sel.first' and 'sel.last', which simplifies what we need in Text._decode.

Experiments indicate that the two sel marks can be set with mark_set, in the sense that the internal structures are set even though the editor screen is not affected:

>>> t.mark_set('sel.first', 1.0)
>>> t.mark_set('sel.last', 1.3)
>>> e. get_selection_indices()
('1.0', '1.3')

'Find in files' indicates that Idle never does this. Except for two instances, the only three marks set are 'insert', 'iomark', and 'my_anchor', which again simplifies mock Text.

Once tests are running with Tk, we can comment out the tk use code (see test_rstrip.py) and add to the mocks what is either needed, or appears reusable for other tests, to make these tests pass again.

For instance, we could subclass mock_tk.Test to add test-specific methods with either 'pass' or minimal specialized code needed.

*mock_idle.Editor:

To use the subclass of Test, one option would be a subclass of mock_idle.Editor that uses the subclass of Text.

Editor.__init__.

Or we could add a parameter text_class to init that defaults to mock_tk.Text, so we can inject a subclass thereof without subclassing Editor.

The two undo_block lines are ok but irrelevant until we can use mock test. You might have mentioned that they are copied from EditorWindow. (The reason they work is that EditorWindow().text is not tkinter.Text() but a Python wrapper thereof.)

The purpose of get_selection_indices is to report the endpoints of the user-selected slice of text. Normal users do so with the mouse. We will have to do so by directly setting the return values. If I did not want to be able to run the rests with tk, the following would be sufficient.

    _selection = ('', '')
    def get_selection_indices(self):
        return self._selection
# with test code setting editor._selection = ('u.v', 'x.y')

However, since that will not work with tk, which as near as I can tell, requires text.mark_set.

mock_tk.Text:

Any (non-trivial) changes to this and its tests should be a separate issue or at least a separate patch.

In any case, the patch is wrong as it is. So are the ones for test_rstrip.py and test_text.py.