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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
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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
