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

Apple GarageBand 10.1 - Complete recordi...
The new GarageBand is a whole music creation studio right inside your Mac -- complete with keyboard, synths, orchestral and percussion instruments, presets for guitar and voice, an entirely... Read more
Duplicate Annihilator 5.7.7 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator... Read more
OS X Server 4.1.3 - For OS X 10.10 Yosem...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
Firefox 39.0 - Fast, safe Web browser. (...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
pwSafe 4.1 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Kodi 15.0.rc1 - Powerful media center to...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Coda 2.5.11 - One-window Web development...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more
Bookends 12.5.7 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Maya 2016 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
RapidWeaver 6.2.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more

Rage of Bahamut is Giving Almost All of...
The App Store isn't what it used to be back in 2012, so it's not unexpected to see some games changing their structures with the times. Now we can add Rage of Bahamut to that list with the recent announcement that the game is severely cutting back... | Read more »
Adventures of Pip (Games)
Adventures of Pip 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** ONE WEEK ONLY — 66% OFF! *** “Adventures of Pip is a delightful little platformer full of charm, challenge and impeccable... | Read more »
Divide By Sheep - Tips, Tricks, and Stre...
Who would have thought splitting up sheep could be so involved? Anyone who’s played Divide by Sheep, that’s who! While we’re not about to give you complete solutions to everything (because that’s just cheating), we will happily give you some... | Read more »
NaturalMotion and Zynga Have Started Tea...
An official sequel to 2012's CSR Racing is officially on the way, with Zynga and NaturalMotion releasing a short teaser trailer to get everyone excited. Well, as excited as one can get from a trailer with no gameplay footage, anyway. [Read more] | Read more »
Grab a Friend and Pick up Overkill 3, Be...
Overkill 3 is a pretty enjoyable third-person shooter that was sort of begging for some online multiplayer. Fortunately the begging can stop, because its newest update has added an online co-op mode. [Read more] | Read more »
Scanner Pro's Newest Update Adds Au...
Scanner Pro is one of the most popular document scanning apps on iOS, thanks in no small part to its near-constant updates, I'm sure. Now we're up to update number six, and it adds some pretty handy new features. [Read more] | Read more »
Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Wars of the Roses (Games)
Wars of the Roses 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
TapMon Battle (Games)
TapMon Battle 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: It's time to battle!Tap! Tap! Tap! Try tap a egg to hatch a Tapmon!Do a battle with another tapmons using your hatched tapmons! *... | Read more »
Alchemic Dungeons (Games)
Alchemic Dungeons 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ### Release Event! ### 2.99$->0.99$ for limited time! ### Roguelike Role Playing Game! ### Alchemic Dungeons is roguelike... | Read more »

Price Scanner via MacPrices.net

RamDisk4Mac App Helps Run Your Mac Faster And...
Ever use a RAM disk? If you’ve come to the Mac in the OS X era, likely not. The Classic Mac OS had a RAM disk function built-in, but that was dropped in the conversion to OS X. What is a RAM disk?... Read more
13-inch 1.6GHz MacBook Air on sale for $849,...
Best Buy has the 2015 13″ 1.6GHz/128GB MacBook Air on sale for $849.99 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Apple Refurbished iMacs available for up to $...
The Apple Store has Apple Certified Refurbished iMacs available for up to $380 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $1949 $... Read more
Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
Seagate Backup Plus Drives Feature 200GB of C...
Seagate Technology plc has announced that its Backup Plus family of external storage offerings will now include 200GB of OneDrive cloud storage, a major added value, and the addition of Lyve’s photo... Read more
Canon PIXMA MG3620 Wireless Inkjet All-in-One...
Canon U.S.A., Inc. has announced the PIXMA MG3620 Wireless (1) Inkjet All-in-One (AIO) printer for high-quality photo and document printing. Built with convenience in mind for the everyday home user... Read more
July 4th Holiday Weekend 13-inch MacBook Pro...
Save up to $150 on the purchase of a new 2015 13″ Retina MacBook Pro at the following resellers this weekend. Shipping is free with each model: 2.7GHz/128GB MSRP $1299 2.7GHz/... Read more
27-inch 3.5GHz 5K iMac on sale for $2149, sav...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2149.99. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary. Their price is $... Read more
Apple now offering refurbished 2015 11-inch...
The Apple Store is now offering Apple Certified Refurbished 2015 11″ MacBook Airs as well as 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.5GHz Retina MacBook Pro on sale for $2274 including free shipping. Their price is $225 off MSRP, and it’s the lowest price available for this model. Read more

Jobs Board

*Apple* Music Producer - Apple (United State...
**Job Summary** Apple Music seeks a Producer to help shepherd some of the most important content and editorial initiatives within the music app, with a particular focus Read more
Editor, *Apple* News - Apple (United States...
**Job Summary** Editor, Apple News The Apple News team is looking for passionate, knowledgeable editors to help identify and deliver the best in breaking national, Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Project Manager - *Apple* Searc...
**Job Summary** Apple 's new Spotlight Suggestions service provides fast, relevant search results from the Inte et in Spotlight and Safari on iOS and OS X. We are looking Read more
Business Development Manager - *Apple* Pay...
**Job Summary** Apple Pay is seeking an experienced relationship manager to support the ongoing management of partners for the Apple Pay platform. This position will Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.