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Augmenting Methods: The Good Way
What we really want to do here is somehow augment the original giveRaise, instead of replacing it altogether. The good way to do that in Python is by calling to the original version directly, with augmented arguments, like this:
class Manager(Person):
def giveRaise(self, percent, bonus=.10):
Person.giveRaise(self, percent + bonus) # Good: augment original
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This code leverages the fact that a class method can always be called either through an instance (the usual way, where Python sends the instance to the self argument automatically) or through the class (the less common scheme, where you must pass the instance manually). In more symbolic terms, recall that a normal method call of this form:
instance.method(args...)
is automatically translated by Python into this equivalent form:
class.method(instance, args...)
where the class containing the method to be run is determined by the inheritance search rule applied to the method’s name. You can code either form in your script, but there is a slight asymmetry between the two—you must remember to pass along the instance manually if you call through the class directly. The method always needs a subject instance one way or another, and Python provides it automatically only for calls made through an instance. For calls through the class name, you need to send an instance to self yourself; for code inside a method like giveRaise, self already is the subject of the call, and hence the instance to pass along.
Calling through the class directly effectively subverts inheritance and kicks the call higher up the class tree to run a specific version. In our case, we can use this technique to invoke the default giveRaise in Person, even though it’s been redefined at the Manager level. In some sense, we must call through Person this way, because a self.giveRaise() inside Manager’s giveRaise code would loop—since self already is a Manager, self.giveRaise() would resolve again to Manager.giveRaise, and so on and so forth until available memory is exhausted.
This “good” version may seem like a small difference in code, but it can make a huge difference for future code maintenance—because the giveRaise logic lives in just one place now (Person’s method), we have only one version to change in the future as needs evolve. And really, this form captures our intent more directly anyhow—we want to perform the standard giveRaise operation, but simply tack on an extra bonus. Here’s our entire module file with this step applied:
# Add customization of one behavior in a subclass
class Person:
def __init__(self, name, job=None, pay=0):
self.name = name
self.job = job
self.pay = pay
def lastName(self):
return self.name.split()[-1]
def giveRaise(self, percent):
self.pay = int(self.pay * (1 + percent))
def __str__(self):
return '[Person: %s, %s]' % (self.name, self.pay)
class Manager(Person):
def giveRaise(self, percent, bonus=.10): # Redefine at this level
Person.giveRaise(self, percent + bonus) # Call Person's version
if __name__ == '__main__':
bob = Person('Bob Smith')
sue = Person('Sue Jones', job='dev', pay=100000)
print(bob)
print(sue)
print(bob.lastName(), sue.lastName())
sue.giveRaise(.10)
print(sue)
tom = Manager('Tom Jones', 'mgr', 50000) # Make a Manager: __init__
tom.giveRaise(.10) # Runs custom version
print(tom.lastName()) # Runs inherited method
print(tom) # Runs inherited __str__
To test our Manager subclass customization, we’ve also added self-test code that makes a Manager, calls its methods, and prints it. Here’s the new version’s output:
[Person: Bob Smith, 0]
[Person: Sue Jones, 100000]
Smith Jones
[Person: Sue Jones, 110000]
Jones
[Person: Tom Jones, 60000]
Everything looks good here: bob and sue are as before, and when tom the Manager is given a 10% raise, he really gets 20% (his pay goes from $50K to $60K), because the customized giveRaise in Manager is run for him only. Also notice how printing tom as a whole at the end of the test code displays the nice format defined in Person’s __str__: Manager objects get this, lastName, and the __init__ constructor method’s code “for free” from Person, by inheritance.
