In a perfect world with perfect coders we wouldn't need tests, things would just work. So tests are a trade off, insurance against our human fallibility. As wit

In a perfect world with perfect coders we wouldn't need tests, things would just work. So tests are a trade off, insurance against our human fallibility. As with other types of insurance you weigh the probability and costs involved in something failing and invest accordingly in insuring it. Covering every edge case isn't necessary. Tests should cover core functionality and then the ongoing addition of regression tests. We hover in the mid to high eighties with our test code coverage and find getting more than that overly time consuming (Python/Django btw). 80/20 rule coming into play?