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CHAPTER 7 ■ ■ ■ More Abstraction In the previous chapters, you looked at Python’s main built-in object types (numbers, strings, lists, tuples, and dictionaries); you peeked at the wealth of built-in functions and standard libraries; you even created your own functions. Now, only one thing seems to be missing— making your own objects. And that’s what you do in this chapter. You may wonder how useful this is. It might be cool to make your own kinds of objects, but what would you use them for? With all the dictionaries and sequences and numbers and strings available, can’t you just use them and make the functions do the job? Certainly. But making your own objects (and especially types or classes of objects) is a central concept in Python—so central, in fact, that Python is called an object-oriented language (along with SmallTalk, C++, Java, and many others). In this chapter, you learn how to make objects, and you learn about polymorphism and encapsulation, methods and attributes, superclasses and inheritance— you learn a lot. So let’s get started. ■Note If you’re already familiar with the concepts of object-oriented programming, you probably know about constructors. Constructors will not be dealt with in this chapter; for a full discussion, see Chapter 9. The Magic of Objects In object-oriented programming, the term object loosely means a collection of data (attributes) with a set of methods for accessing and manipulating those data. There are several reasons for using objects instead of sticking with global variables and functions. Some of the most important benefits of objects include the following: • Polymorphism • Encapsulation • Inheritance Roughly, these terms mean that you can use the same operations on objects of different classes, and they will work as if “by magic” (polymorphism); you hide unimportant details of how objects work from the outside world (encapsulation), and you can create specialized classes of objects from general ones (inheritance). 139
140 CHAPTER 7 ■ MORE ABSTRACTION In many presentations of object-oriented programming, the order of these concepts is different. Encapsulation and inheritance are presented first, and then they are used to model real-world objects. That’s all fine and dandy, but in my opinion, the most interesting feature of object-oriented programming is polymorphism. It is also the feature most people (in my experience) get confused by. Therefore I’ll start with polymorphism, and show that this concept alone should be enough to make you like object-oriented programming. Polymorphism The term polymorphism is derived from a Greek word meaning “having multiple forms.” Basically, that means that even if you don’t know what kind of object a variable refers to, you may still be able to perform operations on it that will work differently depending on the type (or class) of the object. For example, assume that you are creating an online payment system for a commercial Web site that sells food. Your program receives a “shopping cart” of goods from another part of the system (or other similar systems that may be designed in the future)—all you need to worry about is summing up the total and billing some credit card. Your first thought may be to specify exactly how the goods must be represented when your program receives them. For example, you may want to receive them as tuples, like this: ('SPAM', 2.50) If all you need is a descriptive tag and a price, this is fine. But it’s not very flexible. Let’s say that some clever person starts an auctioning service as part of the Web site—where the price of an item is gradually reduced until somebody buys it. It would be nice if the user could put the object in his or her shopping cart and proceed to the checkout (your part of the system) and just wait until the price was right before pressing the Pay button. But that wouldn’t work with the simple tuple scheme. For that to work, the object would have to check its current price (through some network magic) each time your code asked for it—it couldn’t be frozen like in a tuple. You can solve that; just make a function: # Don't do it like this... def getPrice(object): if isinstance(object, tuple): return object[1] else: return magic_network_method(object) ■Note The type/class checking and use of isinstance here is meant to illustrate a point—namely that type checking isn’t generally a satisfactory solution. Avoid type checking if you possibly can. The function isinstance is described in the section “Investigating Inheritance,” later in this chapter. In the preceding code, I use the functions type and isinstance to find out whether the object is a tuple. If it is, its second element is returned; otherwise, some “magic” network method is called.
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Beginning Python From Novice to Pro
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For Ranveig
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■CHAPTER 23 Project 4: In the New
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viii ■CONTENTS Strings . . . . .
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x ■CONTENTS ■CHAPTER 5 Conditio
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xxii ■CONTENTS Preparations . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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CHAPTER 1 ■ ■ ■ Instant Hacki
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CHAPTER 2 ■ ■ ■ Lists and Tup
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CHAPTER 3 ■ WORKING WITH STRINGS
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CHAPTER 4 ■ DICTIONARIES: WHEN IN
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82 CHAPTER 5 ■ CONDITIONALS, LOOP
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CHAPTER 9 ■ MAGIC METHODS, PROPER
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CHAPTER 10 ■ ■ ■ Batteries In
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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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314 CHAPTER 15 ■ PYTHON AND THE W
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CHAPTER 16 ■ ■ ■ Testing, 1-2
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358 CHAPTER 17 ■ EXTENDING PYTHON
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CHAPTER 18 ■ ■ ■ Packaging Yo
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CHAPTER 18 ■ PACKAGING YOUR PROGR
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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CHAPTER 22 ■ PROJECT 3: XML FOR A
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CHAPTER 24 ■ PROJECT 5: A VIRTUAL
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CHAPTER 29 ■ PROJECT 10: DO-IT-YO
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APPENDIX A ■ ■ ■ The Short Ve
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APPENDIX A ■ THE SHORT VERSION 54
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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 577 assertEqual method Tes
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■INDEX 579 cmd module 252, 501 Cm
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■INDEX 581 get method 74-75 has_k
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■INDEX 583 ■F Factory class twi
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■INDEX 585 finding conflicts 197
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■INDEX 587 initialization 38 nami
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■INDEX 589 localtime function tim
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■INDEX 591 nesting blocks 88 Netw
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■INDEX 593 playful programming 38
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■INDEX 595 Python C API 365 hand-
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■INDEX 601 TestCase class methods
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■INDEX 603 further exploration 47
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