My problem is more that I never know what to test. I mean, suppose I have some nontrivial code -- one of my latest random projects contains this:

    def permute(tpl):
        """Gives the set of valid permutations for this tuple."""
        if len(tpl) == 0:
            yield ()
        else:
            for t in permute(tpl[1:]):
                for n in range(0, len(t) + 1):
                    yield t[0:n] + (tpl[0],) + t[n:]

It's a recursive generator-driven permutation engine. Technically I suppose the order of the permutations doesn't matter, so long as they are all distinct and there are n! of them. Is that what I should be testing? That is, should my test code read:

    permute_test = tuple(permute((1, 2, 3, 4)))
    assert(len(permute_test) == 24)
    for i in range(0, 24):
        assert(len(permute_test[i]) == 4)

    for i in range(0, 23):
        for j in range(i + 1, 24):
            assert(permute_test[i] != permute_test[j])

...? And if so, how does that help me write the original function? Or am I supposed to test a base case and one or two recursion steps, so that it helps me write the function, but "hard-wires" a particular order?