I recently discovered the StringIO module and have been using it surprisingly often. It’s not a terribly flashy module, being roughly equivalent to a C++ stringstream, but it is proving useful more than I expected. There are a couple use cases that have come up more than once just in the last couple weeks:

In retrospect, at least the second of these things seems obvious: like it should be a bog-standard way to write unit tests that don’t depend on external resources. But I’ve done a fair amount of cruising StackOverflow and other places looking at Python unit test best practices and haven’t ever stumbled across it before.