Test Your Knowledge: Answers

 

 
  1. 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.
  2. 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.
  3. 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.
  4. 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 object, so their usage is somewhat more complex in this role.
  5. Our chief weapon is decorators...decorators and metaclasses...metaclasses and decorators.... Our two weapons are metaclasses and decorators...and ruthless efficiency.... Our three weapons are metaclasses, decorators, and ruthless efficiency...and an almost fanatical devotion to Guido.... Our four...no....Amongst our weapons.... Amongst our weaponry...are such elements as metaclasses, decorators.... I’ll come in again....