Author Dusty Phillips Reviewers

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Finally, try adding some new features to the three designs. Whatever features strike 
your fancy are fine. I'd like to see a way to differentiate between available and 
unavailable properties, for starters. It's not of much use to me if it's already rented!
Which design is easiest to extend? Which is hardest? If somebody asked you why 
you thought this, would you be able to explain yourself?
Summary
We've gone from simple inheritance, one of the most useful tools in the object-oriented 
programmer's toolbox, all the way through to multiple inheritance, one of the most 
complicated. Inheritance can be used to add functionality to existing classes and built-
ins using inheritance. Abstracting similar code into a parent class can help increase 
maintainability. Methods on parent classes can be called using 
super
and argument 
lists must be formatted safely for these calls to work when using multiple inheritance.
In the next chapter, we'll cover the subtle art of handling exceptional circumstances.
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Expecting the Unexpected
Programs are very fragile. It would be ideal if code always returned a valid result, 
but sometimes a valid result can't be calculated. For example, it's not possible to 
divide by zero, or to access the eighth item in a five-item list.
In the old days, the only way around this was to rigorously check the inputs for every 
function to make sure they made sense. Typically, functions had special return values 
to indicate an error condition; for example, they could return a negative number to 
indicate that a positive value couldn't be calculated. Different numbers might mean 
different errors occurred. Any code that called this function would have to explicitly 
check for an error condition and act accordingly. A lot of code didn't bother to do this, 
and programs simply crashed. However, in the object-oriented world, this is not
the case.
In this chapter, we will study 
exceptions
, special error objects that only need to be 
handled when it makes sense to handle them. In particular, we will cover:
• 
How to cause an exception to occur
• 
How to recover when an exception has occurred
• 
How to handle different exception types in different ways
• 
Cleaning up when an exception has occurred
• 
Creating new types of exception
• 
Using the exception syntax for flow control
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Expecting the Unexpected