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That concept has been formalized as part of the Python standard library.

Ah very nice! Did not realize this was a part of the standard library!

Here is the first announce of doctest I can find (1999) I think it's Python 1.5 time.

Elixir has this.

I use this in my books to show the output but also to "test" that the code found in my books actually works.

I love doctest as it works so well with a REPL but unfortunately it hasn't really gained traction anywhere I've seen.