From the description, doctest seems more like a kind of unit test: it makes sure that the test cases embedded in the doc string return the correct values when run. But you can&#x27;t <i>prove</i> that the a function works correctly just by running a few test cases (or even a lot of test cases) - doctest has no representation of the function, so it can&#x27;t reason about how the function is supposed to work (let alone the system as a whole). For example, it can&#x27;t tell if the arguments passed to this function by another function will always conform to what this function expects to receive.