第631页 | Learning Python | 阅读 ‧ 电子书库

Class Interface Techniques

Extension is only one way to interface with a superclass. The file shown in this section, specialize.py, defines multiple classes that illustrate a variety of common techniques:

Super

Defines a method function and a delegate that expects an action in a subclass.

Inheritor

Doesn’t provide any new names, so it gets everything defined in Super.

Replacer

Overrides Super’s method with a version of its own.

Extender

Customizes Super’s method by overriding and calling back to run the default.

Provider

Implements the action method expected by Super’s delegate method.

Study each of these subclasses to get a feel for the various ways they customize their common superclass. Here’s the file:

class Super:
    def method(self):
        print('in Super.method')           # Default behavior
    def delegate(self):
        self.action()                      # Expected to be defined

class Inheritor(Super):                    # Inherit method verbatim
    pass

class Replacer(Super):                     # Replace method completely
    def method(self):
        print('in Replacer.method')

class Extender(Super):                     # Extend method behavior
    def method(self):
        print('starting Extender.method')
        Super.method(self)
        print('ending Extender.method')

class Provider(Super):                     # Fill in a required method
    def action(self):
        print('in Provider.action')

if __name__ == '__main__':
    for klass in (Inheritor, Replacer, Extender):
        print('\n' + klass.__name__ + '...')
        klass().method()
    print('\nProvider...')
    x = Provider()
    x.delegate()

A few things are worth pointing out here. First, the self-test code at the end of this example creates instances of three different classes in a for loop. Because classes are objects, you can put them in a tuple and create instances generically (more on this idea later). Classes also have the special __name__ attribute, like modules; it’s preset to a string containing the name in the class header. Here’s what happens when we run the file:

% python specialize.py

Inheritor...
in Super.method

Replacer...
in Replacer.method

Extender...
starting Extender.method
in Super.method
ending Extender.method

Provider...
in Provider.action