Classes and Exceptions

Classes allow us to define new types for Python. We can first think of a class as defining a new container – instead of a list, tuple, set, or dictionary, we can have our own collection of values, each with a chosen name rather than an index/key. We can then read out a value or update a value, much like reading or replacing the values in a list or dictionary. But we can also put methods in a class definition, giving us a way to specify the exact ways we should interact with values of this new type. Once we have created a class definition, we can create as many objects of the new type as we want and use them in our programs. We can create entirely new types during the design phase of writing a program. This enables us to think in terms of types (and instances of types) that better model what we see in the real world, instead of endlessly relying on lists or dictionaries (and having to remember exactly how we intended to use those things as proxies for the values we actually had in mind). Exceptions allow us to recover from unusual events – some unavoidable (user types in bad file name or bad input in general), some unexpected (such as IndexErrors when stepping through a list). Without exceptions, we tend to program in an "ask permission, then attempt" style of calculation. But using exceptions, we can instead program in a "try it first, beg for forgiveness" approach by writing code that specifies how to recover from exceptions that we allowed to occur.