TweetFollow Us on Twitter

Mac in the Shell: Python: Getting OOPy with it

Volume Number: 25
Issue Number: 04
Column Tag: Mac in the Shell

Mac in the Shell: Python: Getting OOPy with it

Or, delving into the real Object-Oriented Programming Features of Python

by Edward Marczak

Introduction

The last several articles have been focusing on Python basics: data types, flow control, modules and more. Plus, the first "Learn Python on the Mac" article pointed out some Mac-specific tweaks needed to enable the built-in help docs. Here, we go a little deeper and talk about creating classes, Python's real Object Oriented Programming (OOP) powerhouse. This article will start to introduce OOP in general, and next month's column will get into Python specifics. Without further delay, let's begin.

Why OOP?

Why OOP, indeed? I went back and forth for some time deciding if this was the right time to introduce the topic. There are plenty of other subjects relating to Python that I still haven't addressed. One can write perfectly competent, purely procedural Python programs without ever touching OOP directly. However, understanding OOP and having the class mechanism at your disposal is incredibly powerful. Also, if you ever want to get into GUI applications using Python as the language underneath, the ability to write classes is essential. Finally, those 'other topics' that I mentioned will come along in later articles, so, it will be good to get OOP into your brain sooner rather than later.

Essentially, classes are just another namespace packaging mechanism in Python. Typically, they provide a generic model of a real-world object. Classes promote reusability, and can cut development time. If you're coming to Python from C++ or Java, you're in for a surprise: classes in Python do away with some of the pomp and circumstance found in those languages. Like most things in Python, they're straightforward and easy to understand.

Objects

I've talked a lot about objects in Python in previous article, but we haven't really defined what that means. In one sense, you're using objects in Python if you know it or not. Python generically treats everything as an internal object. For example, when you make the assignment 'x = 'hello', x "is a" string object. On a more pedantic level, though, objects mean several things.

Model Factory

When you define a class, the class itself doesn't really do much. It acts as a model for instantiated objects. It's a factory that can churn out objects based on the namespace it contains.

Inheritance

Classes can inherit methods and objects from other classes.

Extending and Overloading

Related to inheritance, objects can then extended an inherited class by adding new attributes and methods, or, override an attribute or method of the class it inherits from. Extending adds new behavior to the class. Overriding-also called overloading-changes the behavior of the method or class.

It all comes down to the dot operator, which we've been using all along to access module attributes and to call object methods (like a string method to strip whitespace). We've also talked a bit about namespaces. The subtlety behind the dot notation is that it forces python to perform an upward search, through the inheritance tree of namespaces. How does this work?

Last month, we talked about modules, and we can use that initially to illustrate. Take the following example:

#!/usr/bin/env python
x = 5
y = 7
def ModuleA():
  x = 12
  print 'In ModuleA'
  print x
  print y
print 'Main'
print x
ModuleA()

As you may expect, running this short program gives the following output:

Main
5
In ModuleA
12
7

Obvious? Perhaps. The main routine is asked to print x, finds x in its own namespace and does so. Then, we call ModuleA(), which first sets x, and is then asked to print x. Well, at this point, there are two x variables. So, the print statement in ModuleA() starts a search-from the bottom up-to locate x. It first looks in its own namespace, and finds x. Its own version of x, that is, and prints it. ModuleA() is then asked to print y. It again starts a search. Since it doesn't find y in its own namespace, it looks in the namespace directly above it. It finds y there and stops the search.

Note that in the above paragraph, it all comes back to namespaces. In fact, this search-though-the-namespace is initiated any time we use the dot operator in the object.attribute form.

It's all about the dot

As mentioned above, a class is just another Python mechanism of packaging up a namespace. Here's a simple Python class:

class ClassA():
  a = 5
  b = 7

Now, a slightly dirty secret is that a class really is just a namespace package, and we don't even have to instantiate it:

print ClassA.a

will do what you expect and print "5". You can even make further assignments:

ClassA.c = 9
print ClassA.c

Treating a class as a generic, function-like namespace sells the class mechanism a little short, though. Classes have the power to inherit attributes from other classes. Let's define another class:

class ClassB(ClassA):
  b = 44
  c = 88

In this case, ClassB inherits from ClassA. Essentially, all of the attributes from ClassA are copied into ClassB on creation. Let's instantiate three objects to illustrate:

inst_a = ClassA()
inst_b = ClassB()
inst_c = ClassA()

Note that both inst_a and inst_c are instances of ClassA. Each object has a variable a in its namespace:

print inst_a.a
print inst_b.a
print inst_c.a
5
5
5

When inst_b is instantiated, ClassB inherits from ClassA. This is why ClassB has an 'a' variable in its namespace. Figure 1 shows this graphically.


Figure 1: Class inheritance

When ClassB() is asked about variable 'a', it first searches its namespace. Not finding it, the search continues in the class it inherited from-also called its superclass-and the variable is found. We can alter these variables in our instance simply:

inst_b.a = 99
print inst_b.a

This will print 99, as you'd expect. However, it is only changed in the instance, not up the chain:

print ClassB.a
...prints '5'.

Conclusion

Creating an elegant object-oriented program takes some planning. That may be one of the key differences between a straight procedural based program, and an OOP-based one. OOP lets you be a bit more strategic. It also lets you build a larger app based on smaller, reusable objects. Next month, we'll dig into details and implementation a bit more.

Media of the month: "No Line On The Horizon" by U2. Now, this may be an obvious choice, a safe choice, or a polarizing choice. U2 has become pretty palatable to a broad audience these days. If you're one of those people that never gave them a shot, this is actually a decent album to start with.


Ed Marczak is the Executive Editor of MacTech Magazine. He lives in New York with his wife, two daughters and various pets. He has been involved with technology since Atari sucked him in, and has followed Apple since the Apple I days. He spends his days on the Mac team at Google, and free time with his family and/or playing music. Ed is the author of the Apple Training Series book, "Advanced System Administration v10.5," and has written for MacTech since 2004.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

How to become the ultimate robot warrior...
Chrono Strike is a delightfully immersive beat ‘em up with a sense of humor (any game with a good Sims reference gets points in my book). [Read more] | Read more »
Tips and tricks to get a higher score in...
Snow Roll is a devilish endless runner very much in the vein of Flappy Bird. It revels in its dastardly level of difficulty, and doesn’t really care how angry you get at it as it knows you’ll keep coming back for more. [Read more] | Read more »
How to win big in Slots Deluxe
Cheating while gambling is illegal and morally wrong, and in some parts of the world it leads to men with names like Vinnie "Six Knuckles" Manchenzo beating you to a pulp in a dark alley. [Read more] | Read more »
How to take over the world in Dictator 2
Running a country isn't easy - especially when you're a dictator who wants to take over the world and crush everyone in your path while you do it. [Read more] | Read more »
Tips and tricks to get a higher score in...
Tank.iois - you guessed it! - another multiplayer arena battler likeAgar.io and Slither.io. It does differentiate itself by putting you in a tiny tank though, so it's not exactly the same. To help you get that all-important high score, we've got a... | Read more »
How to unlock characters in One Tap Tenn...
As the title suggests, One Tap Tennis requires only a single tap to play its particular brand of tennis, and rewards you with a ton of unlockable characters if you perform well. Fortunately for you, we at 148Apps have got a few tips and tricks to... | Read more »
Grab it now: Game Craft’s Legend of War...
The real time strategy game is now available for you to sink your teeth into, through the App Store and Google Play. Combining elements of skill, strategy and empire building, Legend of War is a real gamers’ game. [Read more] | Read more »
Skateboard Party 3 ft. Greg Lutzka (Gam...
Skateboard Party 3 ft. Greg Lutzka 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Skateboard Party is back! This third edition of the popular sports franchise features professional skater... | Read more »
Cubious (Games)
Cubious 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cubious – How smart are you? How high is your IQube? Solve the impossible puzzles to find out, and help a lost little cube find his... | Read more »
Goat Simulator Waste of Space (Games)
Goat Simulator Waste of Space 1.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.1 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** | Read more »

Price Scanner via MacPrices.net

Enterprise Workers Pick Technology Over Perks...
New Adobe study shows surprising attitudes about office jobs and where the future of work is heading. Adobe has released survey findings revealing that a surprising 70 percent of U.S. office workers... Read more
Memorial Day Weekend Sale: $50-$100 off 11-in...
B&H Photo has 13″ and 11″ MacBook Airs with 256GB SSDs on sale for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/256GB MacBook Air: $999 $100 off MSRP... Read more
Memorial Day Weekend Sales: Apple MacBook Pro...
B&H Photo has 13″ and 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz... Read more
Memorial Day Weekend Sales: Apple iMacs and M...
Take up to $150 off the price of a new iMac or Mac mini at the following Apple resellers this Memorial Day weekend: iMacs: B&H Photo has 21″ and 27″ iMacs on sale for up to $150 off MSRP... Read more
Apple refurbished Retina MacBook Pros availab...
Apple has Certified Refurbished 2015 13″ and 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free... Read more
Apple refurbished 11-inch MacBook Airs availa...
Apple has Certified Refurbished 11″ MacBook Airs (the latest models), available for up to $170 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
Goal Zero and OtterBox Partner to Expand iPh...
Goal Zero, specialists in portable power, have announced a partnership with OtterBox, brand smartphone case protection, to offer the Slide and Slide Plus Batteries as modules compatible with the new... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more

Jobs Board

*Apple* Solutions Consultant - APPLE (United...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
*Apple* Project Engineer - Smart Source Inc...
SmartSource is in need of an Apple Project Engineer for a 12 month contract opportunity in Pittsburg, PA. Role: Apple Project Engineer Location: Pittsburg, PA Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
Service Assistant - *Apple* Chevrolet *App...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive, we Read more
Editor, *Apple* News - APPLE (United States...
Job Summary The Apple News team is looking for a passionate and knowledgeable editor with experience covering entertainment/pop culture and experience running social Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.