At the time of writing this book, a PEP-316, named Programming by Contract for Python, is deferred. This doesn't mean that we cannot implement it in Python, because, as introduced at the beginning of the chapter, this is a general design principle.
Probably the best way to enforce this is by adding control mechanisms to our methods, functions, and classes, and if they rail raise a RuntimeError exception or ValueError. It's hard to devise a general rule for the correct type of exception, as that would pretty much depend on the application in particular. These previously mentioned exceptions are the most common types of exception, but if they don't fit accurately with the problem, creating a custom exception would be the ...
