Time to stretch your wings a bit. I'm going to give you a description of a single function in English. Your job is to copy the description into a new text file, and then add tests that describe all the requirements in a way that the computer can understand and check.

Try to make the doctests so that they're not just for the computer. Good doctests tend to clarify things for human readers as well. By and large, this means that you present them to human readers as examples interspersed with the text.

Without further ado, here is the English description:

The fib(N) function takes a single integer as its only parameter N. If N is 0 or 1, the function returns 1. If N is less than 0, the function raises a ValueError. Otherwise, the function returns the sum of fib(N – 1) and fib(N – 2). The returned value will never be less than 1. A naïve implementation of this function would get very slow as N increased.

I'll give you a hint and point out that the last sentence about the function being slow, isn't really testable. As computers get faster, any test you write that depends on an arbitrary definition of "slow" will eventually fail. Also, there's no good way to test the difference between a slow function and a function stuck in an infinite loop, so there's not much point in trying. If you find yourself needing to do that, it's best to back off and try a different solution.

The next-to-last sentence also provides some difficulty, since to test it completely would require running every positive integer through the fib() function, which would take forever (except that the computer will eventually run out of memory and force Python to raise an exception). How do we deal with this sort of thing, then?

The best solution is to check whether the condition holds true for a random sample of viable inputs. The random.randrange() and random.choice() functions in the Python standard library make that fairly easy to do.