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

Sibelius 2020.6 - Music notation solutio...
Sibelius is the world's best-selling music notation software for Mac. It is as intuitive to use as a pen, yet so powerful that it does most things in less than the blink of an eye. The demo includes... Read more
Bookends 13.4.2 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
OmniGraffle Pro 7.16 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
Drive Genius 6.1.0 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
Tor Browser 9.5 - Anonymize Web browsing...
The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. It contains a modified copy of Firefox that... Read more
VueScan 9.7.28 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
OmniGraffle 7.16 - Create diagrams, flow...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
WALTR 2 2.6.26 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Airmail 4.1 - Powerful, minimal email cl...
Airmail is an mail client with fast performance and intuitive interaction. Support for iCloud, MS Exchange, Gmail, Google Apps, IMAP, POP3, Yahoo!, AOL, Outlook.com, Live.com. Airmail was designed... Read more
Iridient Developer 3.3.12 - Powerful ima...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more

Latest Forum Discussions

See All

Willy Jetman: Astromonkey's Revenge...
Barcelona-based developer Last Chicken Games are set to bring their game Willy Jetman: Astromonkey's Revenge to both iOS and Android on 9th July. The Metroidvania is already available on the likes of PS4, Switch and PC but now mobile folk will be... | Read more »
The 5 Best Mobile Real Time Strategy Gam...
Real-time strategy games feel like they’d be a perfect fit for mobile, but they’re trickier to pull off that you might think. The traditional mold of base-building and micro management can work on touch screens, but needs to be carefully honed so... | Read more »
Using your phone in a protest
I can't write about games today. There is a struggle happening in the streets right now and it needs everyone's attention. Here's some good info on how you can use your iOS device safely amidst a protest. | Read more »
Dungonian is a card-based dungeon crawle...
Dungonian is a card-based dungeon crawler from developer SandFish Games that only recently launched as a free-to-play title. It offers an extensive roster of playable heroes to collect and enemies to take down, and it's available right now for iOS... | Read more »
Steam Link Spotlight - Signs of the Sojo...
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was XCOM: Chimera Squad. Read about how it plays using Steam Link's new mouse and keyboard support over here. | Read more »
Steampunk Tower 2, DreamGate's sequ...
Steampunk Tower 2 is a DreamGate's follow up to their previous tower defence game. It's available now for both iOS and Android as a free-to-play title and will see players defending their lone base by kitting it out with a variety of turrets. [... | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Pokemon Go Fest 2020 will be a virtual e...
Niantic has announced that Pokemon Go Fest will still take place this year although understandably it won't be a physical event. Instead, it will become a virtual celebration and is set to be held on 25th and 26th July. [Read more] | Read more »
Marvel Future Fight's major May upd...
Marvel Future Fight's latest update has now landed, and it sounds like a big one. The focus this time around is on Marvel's Guardians of the Galaxy, and it introduces all-new characters, quests, and uniforms for players to collect. [Read more] | Read more »
SINoALICE, Yoko Taro and Pokelabo's...
Yoko Taro and developer Pokelabo's SINoALICE has now opened for pre-registration over on the App Store. It's already amassed 1.5 million Android pre-registrations, and it's currently slated to launch on July 1st. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple restocks 27″ iMacs, Certified Refurbish...
Apple has restocked Certified Refurbished 2019 27″ iMacs starting at $1529 and up to $350 off the cost of new models. Apple’s one-year warranty is standard, shipping is free, and each iMac features a... Read more
Apple’s new 2020 13″ 4-Core MacBook Airs on s...
B&H Photo has Apple’s new 2020 13″ 4-Core MacBook Airs on sale today for $100 off Apple’s MSRP, only $1199. Expedited shipping is free to many addresses in the US. The MacBook Airs are the same... Read more
New Verizon promo: $150 off any Apple Watch w...
Verizon is offering $150 off any Apple Watch when purchased alongside an iPhone through June 10, 2020. They’re also offering up to $100 on any Apple Watch trade-in. Here are the details: “Get $150... Read more
Last year’s 13″ 2.4GHz MacBook Pros are avail...
Apple has Certified Refurbished 2019 13″ 2.4GHz/256GB 4-Core Touch Bar MacBook Pros available for $1359, $440 off original MSRP. Apple’s one-year warranty is included, shipping is free, and each... Read more
Apple’s new 2020 13″ MacBook Pros on sale for...
Apple reseller Abt Electronics has new 2020 13″ MacBook Pros on sale today for up to $140 off MSRP, starting at $1208. Shipping is free, and most configurations are in stock today. Note that Abt’s... Read more
Apple CEO Reacts To Nationwide Protests Over...
NEWS: 06.03.20 – With the recent death of a black man in the custody of a white police officer igniting outrage among Americans from all walks of life, which resulted in protests and civil unrest... Read more
At up to $420 off MSRP, these Certified Refur...
Apple has Certified Refurbished 2019 16″ MacBook Pros available for up to $420 off the cost of new models, starting at $2039. Each model features a new outer case, shipping is free, and an Apple 1-... Read more
Apple restocks refurbished 3rd generation 12....
Apple restocked select 3rd generation 12.9″ WiFi iPad Pros starting at only $699 and up to $330 off original MSRP. Each iPad comes with a standard Apple one-year warranty, outer cases are new, and... Read more
These wireless carriers are offering Apple’s...
Looking for a deal on Apple’s new iPhone SE? Apple itself offers the unlocked 64GB iPhone SE for $399 or $16.62/month. If you’re willing to try a new carrier, two of Apple’s wireless carriers are... Read more
Save $80 on the 64GB 10.5″ iPad Air with this...
Apple has 10.5″ 64GB WiFi iPad Airs models available for $80 off MSRP, Certified Refurbished. Each iPad comes with Apple’s standard one-year warranty and includes a new outer case. Shipping is free... Read more

Jobs Board

*Apple* Architect - SAIC (United States)
**Description** We are currently seeking a motivated, career and customer oriented Apple Architect to join our team in Washington, DC to begin an exciting and Read more
*Apple* Support Engineer - SAIC (United Stat...
**Description** We are currently seeking a motivated, career and customer oriented Apple Support Engineer to join our team in Washington, DC to begin an exciting and Read more
Perioperative RN - ( *Apple* Hill Surgical C...
Perioperative RN - ( Apple Hill Surgical Center) Tracking Code 60593 Job Description Monday - Friday - Full Time Days Possible Saturdays General Summary: Under the Read more
Senior Practice Manager - *Apple* Hill Eye...
Senior Practice Manager - Apple Hill Eye Center Tracking Code 61713 Job Description Apple Hill Medical Center General Summary: Under general supervision, manages Read more
*Apple* Mac Desktop Support - Global Dimensi...
…Operate and support an Active Directory (AD) server-client environment for all Apple devices operating on the BUMED network + Leverage necessary industry enterprise Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.