Beginning Python - From Novice to Professional

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CHAPTER 7 ■ MORE ABSTRACTION 157 When determining which classes you need and what methods they should have, you may try something like this: 1. Write down a description of your problem (what should the program do?). Underline all the nouns, verbs, and adjectives. 2. Go through the nouns, looking for potential classes. 3. Go through the verbs, looking for potential methods. 4. Go through the adjectives, looking for potential attributes. 5. Allocate methods and attributes to your classes. Now you have a first sketch of an object-oriented model. You may also want to think about what relationships (such as inheritance) the classes and objects will have. To refine your model, you can do the following: 6. Write down (or dream up) a set of use cases—scenarios of how your program may be used. Try to cover all the functionality. 7. Think through every use case step by step, making sure that everything you need is covered by your model. If something is missing, add it. If something isn’t quite right, change it. Continue until you are satisfied. When you have a model you think will work, you can start hacking away. Chances are you’ll have to revise your model—or revise parts of your program. Luckily, that’s easy in Python, so don’t worry about it. Just dive in. (If you’d like some more guidance in the ways of objectoriented programming, check out the list of suggested books in Chapter 19.) A Quick Summary This chapter has given you more than just information about the Python language; it has introduced you to several concepts that may have been completely foreign to you. Let me try to summarize them for you: Objects. An object consists of attributes and methods. An attribute is merely a variable that is part of an object, and a method is more or less a function that is stored in an attribute. One difference between (bound) methods and other functions is that methods always receive the object they are part of as their first argument, usually called self. Classes. A class represents a set (or kind) of objects, and every object (instance) has a class. The class’s main task is to define the methods its instances will have. Polymorphism. Polymorphism is the characteristic of being able to treat objects of different types and classes alike—you don’t have to know which class an object belongs to in order to call one of its methods. Encapsulation. Objects may hide (or encapsulate) their internal state. In some languages this means that their state (their attributes) is only available through their methods. In
158 CHAPTER 7 ■ MORE ABSTRACTION Python, all attributes are publicly available, but programmers should still be careful about accessing an object’s state directly, since they might unwittingly make the state inconsistent in some way. Inheritance. One class may be the subclass of one or more other classes. The subclass then inherits all the methods of the superclasses. You can use more than one superclass, and this feature can be used to compose orthogonal pieces of functionality. A common way of implementing this is using a core superclass along with one or more mix-in superclasses. Interfaces and introspection. In general, you don’t want to prod an object too deeply. You rely on polymorphism, and call the methods you need. However, if you want to find out what methods or attributes an object has, there are functions that will do the job for you. Object-oriented design. There are many opinions about how (or whether!) to do objectoriented design. No matter where you stand on the issue, it’s important to understand your problem thoroughly, and to create a design that is easy to understand. New Functions in This Chapter Function callable(object) getattr(object, name[, default]) hasattr(object, name) isinstance(object, class) issubclass(A, B) random.choice(sequence) setattr(object, name, value) type(object) Description Determines if the object is callable (such as a function or a method) Gets the value of an attribute, optionally providing a default Determines if the object has the given attribute Determines if the object is an instance of the class Determines if A is a subclass of B Chooses a random element from a non-empty sequence Sets the given attribute of the object to value Returns the type of the object What Now? You’ve learned a lot about creating your own objects and how useful that can be. Before diving headlong into the magic of Python’s special methods (Chapter 9), let’s take a breather with a little chapter about exception handling.
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Beginning Python From Novice to Pro
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Introduction A C program is like a
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256 CHAPTER 11 ■ FILES AND STUFF
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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358 CHAPTER 17 ■ EXTENDING PYTHON
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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■INDEX 595 Python C API 365 hand-
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