Beginning Python - From Novice to Professional

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CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS 183 implemented (see the section “Emulating Container Types” in the Python Reference Manual, http://www.python.org/doc/ref/sequence-types.html), including the __iter__ method, which I describe in the section “Iterators,” later in this chapter. Implementing all these methods (to make your objects fully polymorphically equivalent to lists or dictionaries) is a lot of work and hard to get right. If you want custom behavior in only one of the operations, it makes no sense that you should have to reimplement all of the others. It’s just programmer laziness (also called common sense). So what should you do? The magic word is “inheritance.” Why reimplement all of these things when you can inherit them? The standard library comes with three ready-to-use implementations of the sequence and mapping protocols (see the sidebar “UserList, UserString, and UserDict”), and in newer versions of Python, you can subclass the built-in types themselves. (Note that this is mainly useful if your class’s behavior is close to the default. If you have to reimplement most of the methods, it might be just as easy to write a new class.) USERLIST, USERSTRING, AND USERDICT The standard library contains three modules called UserList, UserString, and UserDict, each containing a class with the same name as the module. These classes satisfy all the requirements of the sequence and mapping protocols. UserList and UserString are custom sequences that behave just like ordinary lists and strings, while UserDict is a custom mapping that behaves just like ordinary dictionaries. Until Python 2.2, these were the best option as superclasses when creating your own mappings and sequences. In Python 2.2, the capability to subclass built-in types was added, making these less useful. So, if you want to implement a sequence type that behaves similarly to the built-in lists, you can simply subclass list. ■Note When you subclass a built-in type such as list, you are indirectly subclassing object. Therefore your class is automatically new-style, which means that such features as the super function are available. Let’s just do a quick example—a list with an access counter: class CounterList(list): def __init__(self, *args): super(CounterList, self).__init__(*args) self.counter = 0 def __getitem__(self, index): self.counter += 1 return super(CounterList, self).__getitem__(index) The CounterList class relies heavily on the behavior of its subclass (list). Any methods not overridden by CounterList (such as append, extend, index, and so on) may be used directly. In the two methods that are overridden, super is used to call the superclass version of the method, only
184 CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS adding the necessary behavior of initializing the counter attribute (in __init__) and updating the counter attribute (in __getitem__). ■Note Overriding __getitem__ is not a bullet-proof way of trapping user access because there are other ways of accessing the list contents, such as through the pop method. Here is an example of how CounterList may be used: >>> cl = CounterList(range(10)) >>> cl [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> cl.reverse() >>> cl [9, 8, 7, 6, 5, 4, 3, 2, 1, 0] >>> del cl[3:6] >>> cl [9, 8, 7, 3, 2, 1, 0] >>> cl.counter 0 >>> cl[4] + cl[2] 9 >>> cl.counter 2 As you can see, CounterList works just like list in most respects. However, it has a counter attribute (initially zero), which is incremented each time you access a list element. After performing the addition cl[4] + cl[2], the counter has been incremented twice, to the value 2. More Magic There are special (magic) names for many purposes—what I’ve shown you so far is just a small taste of what is possible. Most of the magic methods available are meant for fairly advanced use, so I won’t go into detail here. However, if you are interested, it is possible to emulate numbers, make objects that can be called as if they were functions, influence how objects are compared, and much more. For more information on which magic methods are available, see the section “Special Method Names” in the Python Reference Manual (http://www.python.org/doc/ref/ specialnames.html). Properties In Chapter 7, I mentioned accessor methods. Accessors are simply methods with names such as getHeight and setHeight and are used to retrieve or rebind some attribute (which may be
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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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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 2 ■ ■ ■ Lists and Tup
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110 CHAPTER 6 ■ ABSTRACTION But w
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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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358 CHAPTER 17 ■ EXTENDING PYTHON
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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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 579 cmd module 252, 501 Cm
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■INDEX 589 localtime function tim
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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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