Learning Python - Mark Lutz [579]
Along the way, we also saw how the roles of class decorators and metaclasses often intersect: because both run at the conclusion of a class statement, they can sometimes be used interchangeably. Class decorators can be used to manage both class and instance objects; metaclasses can, too, although they are more directly targeted toward classes.
Since this chapter covered an advanced topic, we’ll work through just a few quiz questions to review the basics (if you’ve made it this far in a chapter on metaclasses, you probably already deserve extra credit!). Because this is the last part of the book, we’ll forego the end-of-part exercises. Be sure to see the appendixes that follow for pointers on installation steps, and the solutions to the prior parts’ exercises.
Once you finish the quiz, you’ve officially reached the end of this book. Now that you know Python inside and out, your next step, should you choose to take it, is to explore the libraries, techniques, and tools available in the application domains in which you work. Because Python is so widely used, you’ll find ample resources for using it in almost any application you can think of—from GUIs, the Web, and databases to numeric programming, robotics, and system administration.
This is where Python starts to become truly fun, but this is also where this book’s story ends, and others’ begin. For pointers on where to turn after this book, see the list of recommended follow-up texts in the Preface. Good luck with your journey. And of course, “Always look on the bright side of Life!”
Test Your Knowledge: Quiz
What is a metaclass?
How do you declare the metaclass of a class?
How do class decorators overlap with metaclasses for managing classes?
How do class decorators overlap with metaclasses for managing instances?
Would you rather count decorators or metaclasses amongst your weaponry? (And please phrase your answer in terms of a popular Monty Python skit.)
Test Your Knowledge: Answers
A metaclass is a class used to create a class. Normal classes are instances of the type class by default. Metaclasses are usually subclasses of the type class, which redefines class creation protocol methods in order to customize the class creation call issued at the end of a class statement; they typically redefine the methods __new__ and __init__ to tap into the class creation protocol. Metaclasses can also be coded other ways—as simple functions, for example—but they are responsible for making and returning an object for the new class.
In Python 3.0 and later, use a keyword argument in the class header line: class C(metaclass=M). In Python 2.X, use a class attribute instead: __metaclass__ = M. In 3.0, the class header line can also name normal superclasses (a.k.a. base classes) before the metaclass keyword argument.
Because both are automatically triggered at the end of a class statement, class decorators and metaclasses can both be used to manage classes. Decorators rebind a class name to a callable’s result and metaclasses route class creation through a callable, but both hooks can be used for similar purposes. To manage classes, decorators simply augment and return the original class objects. Metaclasses augment a class after they create it.
Because both are automatically triggered at the end of a class statement, class decorators and metaclasses can both be used to manage class instances, by inserting a wrapper object to catch instance creation calls. Decorators may rebind the class name to a callable run on instance creation that retains the original class object. Metaclasses can do the same, but they must also create the class