Is there any necessity at all for the IDLE to test the validity of the filenames? 

I mean: the file specification is provided by the underlying operating system, so by definition it has to be taken as valid. Testing for its validity is superfluous and, in my opinion, a fault in the general conception of the procedure. IDLE's only role should be to present the data it receives in the best possible form and silently integrate the election made by the user.

Excuse me if I speak nonsense, but I got this idea from studying *Autocomplete.py* and its dependencies. It seems to me that the difficulty resides in submitting natural language strings to the judgement of *PyParse* and *Hyperparser*, which are not competent for it because its purpose is to check the validity of Python code and Python *identifiers*. This confussion is what in my opinion causes this bug (and probably others related to the treatment of natural language text files by the IDLE, as it could also happen with issue 14929).