Error throwing requirements are too finicky for Python, I recommend loosening the requirements or updating the description. I was missing a period in "This program only works for text." and the test failures didn't provide me enough information to glean that it was the sentence and not the error type that was amiss. Since the instructions only explicitly mention an error type for Groovy, I found that part needlessly confusing; I couldn't tell if it was a translation issue or my own.

It's a nice story.

But do we really need another "replace such-and-such letter" kata?