Ubuntu Unleashed, Second Edition

By default, Python gives you all the methods the parent class has, but you can override

that by providing new implementations for the classes that need them. For example:

class modelt(car):

def setColor(self, color):

print “Sorry, Model Ts only come in black!”

myford = ford()

ford.setColor(“green”)

mycar = modelt()

mycar.setColor(“green”)

The first car is created as a Ford, so setColor() will work fine because it uses the method

from the

car class. However, the second car is created as a Model T, which has its own

setColor() method, so the call will fail.

This provides an interesting scenario: What do you do if you have overridden a method

and yet want to call the parent’s method also? If, for example, changing the color of a

Model T was allowed but just cost extra, we would want to print a message saying, “You

owe $50 more,” but then change the color. To do this, we need to use the class object

from which our current class is inherited—

car in this example. Here’s an example:

class modelt(car):

def setColor(self, color):

print “You owe $50 more”

car.setColor(self, color)

mycar = modelt()

mycar.setColor(“green”)

print mycar.color

That prints the message and then changes the color of the car.

The Standard Library and the Vaults of Parnassus

A default Python install includes many modules (blocks of code) that enable you to inter-

act with the operating system, open and manipulate files, parse command-line options,

perform data hashing and encryption, and much more. This is one of the reasons most

commonly cited when people are asked why they like Python so much—it comes stocked

to the gills with functionality you can take advantage of immediately. In fact, the number

of modules included in the Standard Library is so high that entire books have been

written about them—try Python Standard Library (O’Reilly, ISBN: 0-596-00096-0) for a

comprehensive, if slightly dated, list of them.

For unofficial scripts and add-ons for Python, the recommended starting place is called

the Vaults of Parnassus: http://py.vaults.ca. There you can find about 20,000 public scripts

and code examples for everything from mathematics to games.

Object Orientation

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