TweetFollow Us on Twitter

Mac in the Shell: Learning Python on the Mac: Classes

Volume Number: 25
Issue Number: 05
Column Tag: Mac in the Shell

Mac in the Shell: Learning Python on the Mac: Classes

Building a Basic Class

by Edward Marczak

Introduction

Last month, we covered conceptually what classes are, why they're useful and when you may use them. This was all done with no (real) code. None of the nitty-gritty. That's where we're headed this month. So, without further ado, let's get into some Python code!

Modeling the Real World

Traditionally, defining a class is the mechanism that allows the code author to create a "factory" for churning out (instantiating) objects. Python is a little different in that everything is treated as an object whether you are aware of that fact or not. As we discussed last month, you use a class when you have a model in the real world that you'd like to follow. In his Road to Code column, Dave Dribin has been using shapes as objects. A single shape class can be used to model shapes from the real world: a square, a rectangle, etc. We're going to use a different example that's just as grounded in the real world: a bank account.

Before getting into the code, it's wise to plan out a class: what is it's structure? A class can hold instance variables and methods. An instance variable is simply a variable that is specific to a given class. Similarly, a method is a function that resides internal to a class, and can act on instance variables stored in a class.

What do we need to plan out our bank account class? Well, the account should have some method of identifying it - a name or number, perhaps. Since this is a small example, and we're not worried about name conflicts, and we don't want to treat people like a number, let's just go with name. That's one, actually two, instance variables: First Name and Last Name. The account will also have a balance, our third instance variable.

What actions do we need to take these variables? Focusing on the balance, we certainly need to deposit money. That's our first method! We'll also want to withdraw this money at some point, which will be our second method. Let's start modeling the class from here.

Bank Account Class

First things first: create a directory for this project. I'm using "Bank_Class", but you're free to call it what you wish. Inside that directory, I'm creating a file named "Bank_Class.py". Use vi, BBEdit or whichever plain text editor makes you most comfortable.

First thing is first; the magic shebang line:

#!/usr/bin/env python

As shown last month, classes always begin with the class keyword to define them:

class account:

and we said we need three instance variables: balance, first name and last name. We can define them here (but please read on as we're going to refine this!):

class account:
  balance = 0
  fname = ''
  lname = ''

From here, we can actually use this class:

acct = Account()  # Instantiate a new Account
print 'Account balance is', acct.balance
print 'Depositing $50'
acct.balance = acct.balance + 50
print 'Balance is now', acct.balance

Which yields the output:

Account balance is 0
Depositing $50
Balance is now 50

Now, just because we can doesn't mean that we should! This code will work, but it has a few holes. One large issue being that we don't have to assign a name to this account, and this is what we hoped to be our identifier. If there are actions we want to take, including assignment, every time we create an instance, we can define a constructor. A constructor is just another function (or, method). What makes it special is that it will run every time the class is instantiated. Defining a method named "__init__" creates a constructor (that's two underscores and the word 'init' followed by two underscores. Instead of the small tangled mess shown above, we can define the following:

class Account:
  def __init__(self, fn, ln):
    self.balance=0
    self.fname=fn
    self.lname=ln

Now when we instantiate the class, we can call it like this:

acct1 = Account('Bill', 'Smith')

This will create the account with a zero balance and assign the first name as "Bill" and the last name as "Smith". If we forget one or both parameters, the class will raise an error.

What's with the "self"?

Python requires that there be an additional first parameter to a class method. While you could technically name it anything you like, it's canonically called self. The Python runtime will automatically supply the value for this parameter at runtime. The self parameter is an object reference used to pass instance values to the method. While you can call it anything you like, all of the Python documentation uses "self," along with, well, every Python author that I know or have seen. So, stick with the convention of "self." It'll help you, or anyone that needs to look at your code in the future.

Additional Methods

Now that we can instantiate a new bank account, we'll want to act on it. We've already decided that we need at least two methods: deposit and withdraw. Add the methods to the class (remember the right indentation!):

class Account:
  def __init__(self, fn, ln):
    self.balance = 0
    self.fname = fn
    self.lname = ln
    
  def Deposit(self,amount):
    self.balance += amount
  
  def Withdraw(self, amount):
    self.balance -= amount

Now we can create a new account, deposit and withdraw money. (Note the use of the += and -= operators. This is simple shorthand for repeating the left-hand variable. x = x + 1 can become x += 1). Unlike the first version of this code shown above, we don't have to set the variables ourselves, but rather we use a method to do it for us:

acct = Account('Joan', 'Smith')    # Create new account
print "Acct1 Balance = ", acct1.balance
print "Depositing $50 to acct1"
acct1.Deposit(50)       # Note use of class method here
print "Acct1 Balance =", acct1.balance

Naturally, there are some holes with this. There are no sanity checks to see if there's any money in the account before we withdraw it, for one. That is an improvement left to the reader.

How is this better?

Well, the examples given thus far haven't done much to improve on traditional procedural programming. However, now that we have the structure in place, it's easy to go beyond that. Creating multiple accounts, for instance is as simple as an assignment:

acct1 = Account('Joan', 'Smith')
acct2 = account('Bob', 'Smith')

And we can perform discreet actions on each:

acct1.Deposit(50)
acct2.Deposit(1000)
print "Acct1 Balance =", acct1.balance
print "Acct2 Balance =", acct2.balance

You should be able to visualize a dictionary structure filled with accounts. Or, the ability to find a record in a database and loading the found record(s) into an Account class.

Conclusion

Between last month and this month - please ensure that you also understand the material presented last month, too! - you should have a pretty good idea what classes are, how they work, and how to start building your own. Next month, we'll get into some OS X-specific functionality of Python and build some useful classes.

Media of the month: http://www.facebook.com. Seriously. OK, pick any social network, but Facebook seems to be the biggest. And then go say hello to someone you miss.

Hope to see everyone at WWDC next month! See you in San Francisco!


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

Deeper 2.2.1 - Enable hidden features in...
Deeper is a personalization utility for macOS which allows you to enable and disable the hidden functions of the Finder, Dock, QuickTime, Safari, iTunes, login window, Spotlight, and many of Apple's... Read more
Spotify 1.0.59.395. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
FileZilla 3.27.0.1 - Fast and reliable F...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.27.0.1: MSW: Add misssing file to .zip binary package... Read more
Sierra Cache Cleaner 11.0.6 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Live Home 3D Pro 3.1.2 - $69.99
Live Home 3D Pro, a successor of Live Interior 3D, is the powerful yet intuitive home design software that lets you build the house of your dreams right on your Mac. It has every feature of Live Home... Read more
Pinegrow 3.04 - Mockup and design webpag...
Pinegrow (was Pinegrow Web Designer) is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation,... Read more
DiskCatalogMaker 7.1.2 - Catalog your di...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
Apple iTunes 12.6.2 - Play Apple Music a...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Transmit 5.0 - Excellent FTP/SFTP client...
Transmit is an excellent FTP (file transfer protocol), SFTP, S3 (Amazon.com file hosting) and iDisk/WebDAV client that allows you to upload, download, and delete files over the internet. With the... Read more
Adobe Lightroom 6.12 - Import, develop,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more

Latest Forum Discussions

See All

The best deals on the App Store this wee...
There are quite a few truly superb games on sale on the App Store this week. If you haven't played some of these, many of which are true classics, now's the time to jump on the bandwagon. Here are the deals you need to know about. [Read more] | Read more »
Realpolitiks Mobile (Games)
Realpolitiks Mobile 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: PLEASE NOTE: The game might not work properly on discontinued 1GB of RAM devices (iPhone 5s, iPhone 6, iPhone 6 Plus, iPad... | Read more »
Layton’s Mystery Journey (Games)
Layton’s Mystery Journey 1.0.0 Device: iOS Universal Category: Games Price: $15.99, Version: 1.0.0 (iTunes) Description: THE MUCH-LOVED LAYTON SERIES IS BACK WITH A 10TH ANNIVERSARY INSTALLMENT! Developed by LEVEL-5, LAYTON’S... | Read more »
Full Throttle Remastered (Games)
Full Throttle Remastered 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Originally released by LucasArts in 1995, Full Throttle is a classic graphic adventure game from industry legend Tim... | Read more »
Stunning shooter Morphite gets a new tra...
Morphite is officially landing on iOS in September. The game looks like the space shooter we've been needing on mobile, and we're going to see if it fits the bill quite shortly. The game's a collaborative effort between Blowfish Studios, We're Five... | Read more »
Layton's Mystery Journey arrives to...
As you might recall, Layton's Mystery Journey is headed to iOS and Android -- tomorrow! To celebrate the impending launch, Level-5's released a new trailer, complete with an adorable hamster. [Read more] | Read more »
Sidewords (Games)
Sidewords 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Grab a cup of coffee and relax with Sidewords. Sidewords is part logic puzzle, part word game, all original. No timers. No... | Read more »
Noodlecake Games' 'Leap On!...
Noodlecake Games is always good for some light-hearted arcade fun, and its latest project, Leap On! could carry on that tradition. It's a bit like high stakes tetherball in a way. Your job is to guide a cute little blob around a series of floating... | Read more »
RuneScape goes mobile later this year
Yes, RuneScape still exists. In fact, it's coming to iOS and Android in just a few short months. Jagex, creators of the hit fantasy MMORPG of yesteryear, is releasing RuneScape Mobile and Old School RuneScape for mobile devices, complete with... | Read more »
Crash of Cars wants you to capture the c...
Crash of Cars is going full on medieval in its latest update, introducing castles and all manner of new cars and skins fresh from the Dark Ages. The update introduces a new castle-themed map (complete with catapults) and a gladiator-style battle... | Read more »

Price Scanner via MacPrices.net

Save or Share
FotoJet Designer, is a simple but powerful new graphic design apps available on both Mac and Windows. With FotoJet Designer’s 900+ templates, thousands of resources, and powerful editing tools you... Read more
Logo Maker Shop iOS App Lets Businesses Get C...
A newly released app is designed to help business owners to get creative with their branding by designing their own logos. With more than 1,000 editable templates, Logo Maker Shop 1.0 provides the... Read more
Sale! New 15-inch MacBook Pros for up to $150...
Amazon has the new 2017 15″ MacBook Pros on sale for up to $150 off MSRP including free shipping: – 15″ 2.8GHz MacBook Pro Space Gray: $2249 $150 off MSRP – 15″ 2.89Hz MacBook Pro Space Gray: $2779 $... Read more
DEVONthink To Go 2.1.7 For iOS Brings Usabili...
DEVONtechnologies has updated DEVONthink To Go, the iOS companion to DEVONthink for Mac, with enhancements and bug fixes. Version 2.1.7 adds an option to clear the Global Inbox and makes the grid... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more
13-inch 2.3GHz Silver MacBook Pro on sale for...
B&H Photo has the new 2017 13″ 2.3GHz/256GB Silver MacBook Pro (MPXU2LL/A) on sale for $1399 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
Apple Tackles Distracted Driving With iOS 11...
One of the most important new features coming in iOS 11 is Do Not Disturb while driving, intended to help drivers stay more focused on the road. With Do Not Disturb while driving, your iPhone can... Read more
iMazing Mini for Mac: Free Automatic and Priv...
Geneva, Switzerland-based indie developer DigiDNA has released iMazing Mini, their free macOS utility designed to automatically back up iOS devices over any local Wi-Fi network. The app offers users... Read more
Clearance 2016 13-inch MacBook Airs, Apple re...
Apple dropped prices recently on Certified Refurbished 2016 13″ MacBook Airs, with models now available starting at $809. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
9.7-inch 2017 iPads available for $299, save...
B&H Photo has 2017 9.7″ 32GB WiFi iPads on sale for $30 off MSRP for a limited time. Shipping is free, and pay sales tax in NY & NJ only: – 32GB iPad WiFi: $299, $30 off Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Payments Architect - *Apple* Pay - A...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
Manager, *Apple* Media Products - Apple Inc...
Job Summary The Apple Media Products Discovery, Fraud and Abuse team is responsible for protecting the integrity of Apple services. As a manager of the team, you Read more
*Apple* Watch, Accessories, Engineering Proj...
Job Summary Engineering Project Manager, Apple Watch Accessories. The Accessories group is looking for an Engineering Project Manager (EPM) to lead the design and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.