One of the goals of object-oriented design is to make software more maintainable, which means that you can keep the program working when other parts of the system change, and modify the program to meet new requirements.
A design principle that helps achieve that goal is to keep interfaces separate from implementations. For objects, that means that the methods a class provides should not depend on how the attributes are represented.
For example, in this chapter we developed a class that represents a time of day. Methods provided by this class include time_to_int, is_after, and add_time.
We could implement those methods in several ways. The details of the implementation depend on how we represent time. In this chapter, the attributes of a Time object are hour, minute, and second.
As an alternative, we could replace these attributes with a single integer representing the number of seconds since midnight. This implementation would make some methods, like is_after, easier to write, but it makes some methods harder.
After you deploy a new class, you might discover a better implementation. If other parts of the program are using your class, it might be time-consuming and error-prone to change the interface.
But if you designed the interface carefully, you can change the implementation without changing the interface, which means that other parts of the program don’t have to change.
Keeping the interface separate from the implementation means that you have to hide the attributes. Code in other parts of the program (outside the class definition) should use methods to read and modify the state of the object. They should not access the attributes directly. This principle is called information hiding; see http://en.wikipedia.org/wiki/ Information_hiding.
Exercise 17.6.Download the code from this chapter(http: // thinkpython. com/ code/ Time2. py ). Change the attributes of Time to be a single integer representing seconds since midnight. Then modify the methods (and the function int_to_time) to work with the new implementation. You should not have to modify the test code in main. When you are done, the output should be the same as before. Solution: http: // thinkpython. com/ code/ Time2_ soln. py