Beginning Python - From Novice to Professional

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CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS 201 A Quick Summary A lot of magic here. Let’s take stock: New-style classes. The way classes work in Python is changing. In Python 2.2, new-style classes were introduced, and they provide several new features (for example, they work with super and property, while old-style classes do not). To create a new-style class you must subclass object, either directly or indirectly, or set the __metaclass__ property. Magic methods. Several special methods (with names beginning and ending with double underscores) exist in Python. These methods differ quite a bit in function, but most of them are called automatically by Python under certain circumstances. (For example, __init__ is called after object creation.) Constructors. These are common to many object-oriented languages, and you’ll probably implement one for almost every class you write. Constructors are named __init__ and are automatically called right after an object is created. Overriding. A class can override methods (or any other attributes) defined in its superclasses simply by implementing the methods. If the new method needs to call the overridden version, it can either call the unbound version from the superclass directly (old-style classes) or use the super function (new-style classes). Sequences and mappings. Creating a sequence or mapping of your own requires implementing all the methods of the sequence and mapping protocols, including such magic methods as __getitem__ and __setitem__. By subclassing list (or User List) and dict (or UserDict) you can save a lot of work. Iterators. An iterator is simply an object that has a next method. Iterators can be used to iterate over a set of values. When there are no more values, the next method should raise a StopIteration exception. Iterable objects have an __iter__ method, which returns an iterator, and can be used in for loops, just like sequences. Often, an iterator is also iterable, that is, it has an __iter__ method returning the iterator itself. Generators. A generator function (or method) is a function (or method) that contains the keyword yield. When called, the generator function returns a generator, which is a special type of iterator. Eight Queens. The Eight Queens problem is well known in computer science and lends itself easily to implementation with generators. The goal is to position eight queens on a chess board so that none of the queens is in a position from which she can attack any of the others.
202 CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS New Functions in This Chapter Function iter(obj) property(fget, fset, fdel, doc) super(class, obj) Description Extracts an iterator from an iterable object. Returns a property. All arguments are optional. Returns a bound instance of class’s superclass. Note that iter and super may be called with other parameters than those described here. For more information, see the standard Python documentation (http://python.org/doc). What Now? Now you know most of the Python language. So why are there still so many chapters left, you ask? Oh, there is still a lot to learn, much of it about how Python can connect to the external world in various ways. And then you have the projects . . . So we’re not done yet. Not by far.
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Beginning Python From Novice to Pro
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About the Technical Reviewer ■JER
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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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358 CHAPTER 17 ■ EXTENDING PYTHON
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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