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

beaTunes 4.6.13 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
Vienna 3.1.9 :e81515b: - RSS and Atom ne...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
iExplorer 4.0.12.0 - View and transfer f...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
OpenEmu 2.0.5 - Open Source game-emulati...
OpenEmu is about to change the world of video game emulation, one console at a time... For the first time, the 'It just works' philosophy now extends to open source video game emulation on the Mac.... Read more
TextSoap 8.3.3 - Automate tedious text d...
TextSoap can automatically remove unwanted characters, fix up messed up carriage returns, and do pretty much anything else that we can think of to text. Save time and effort. Be more productive. Stop... Read more
Apple iTunes 12.6 - Play Apple Music and...
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
Airmail 3.2.4 - Powerful, minimal email...
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
MacPilot 9.0.6 - Enable over 1,200 hidde...
MacPilot gives you the power of UNIX and the simplicity of Macintosh, which means a phenomenal amount of untapped power in your hands! Use MacPilot to unlock over 1,200 features, and access them all... Read more
Jamf Pro 9.98 - Powerful sysadmin/enterp...
Jamf Pro (formerly Casper Suite) is the EMM tool that delights IT pros and the users they support by delivering on the promise of unified endpoint management for Apple devices. At Jamf, connecting... Read more
PopChar 7.7 - $16.99 (51% off)
PopChar helps you get the most out of your font collection. With its crystal-clear interface, PopChar provides a frustration-free way to access any font's special characters. Features Expanded... Read more

Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »
Death Road to Canada (Games)
Death Road to Canada 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Get it now at the low launch price! Price will go up a dollar every major update. Update news at the bottom of this... | Read more »
Bean's Quest Beginner's Guide:...
Bean's Quest is a new take on both the classic platformer and the endless runner, and it's free on the App Store for the time being. Instead of running constantly, you can't stop jumping. That adds a surprising new level of challenge to the game... | Read more »
How to rake in the cash in Bit City
Our last Bit City guide covered the basics. Now it's time to get into some of the more advanced techniques. In the later cities, cash flow becomes much more difficult, so you'll want to develop some strategies if you want to complete each level.... | Read more »
PixelTerra (Games)
PixelTerra 1.1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1.1 (iTunes) Description: The world of PixelTerra is quite dangerous so you need to build a shelter, find some food supply and get ready to protect... | Read more »
Tokaido™ (Games)
Tokaido™ 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Discover the digital adaptation of Tokaido, the boardgame phenomenon that has already sold more than 250,000 copies worldwide, and... | Read more »
Card Thief (Games)
Card Thief 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Card Thief is a solitaire style stealth game played with a deck of cards. In Card Thief you move through a deck of cards as a... | Read more »
Smilegate’s crafting battler Super Tank...
Super Tank Rumbleputs you in the seat of your very own, handcrafted tank. You can choose from over 100 different parts to create your Super Tank before taking it out to wreak havoc on your opponents in glorious PVP combat. Now, Smilegate is upping... | Read more »

Price Scanner via MacPrices.net

Save $230 with Apple Certified Refurbished 13...
Apple is now offering Certified Refurbished 2016 13″ 2.0GHz non-Touch Bar MacBook Pros for $230 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
13-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the Apple 13″ Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar... Read more
Today only! 15-inch 2.7GHz Space Gray Touch B...
B&H Photo has the new 2016 15″ 2.7GHz Space Gray Apple Touch Bar MacBook Pro in stock today and on sale for $300 off MSRP for today only. Shipping is free, and B&H charges NY sales tax only... Read more
New $329 iPad A Fabulous Value; 10.5-Inch iPa...
Part of the iPad upgrade/new model puzzle is now in place. Yesterday, as KGI Securities financial services group analyst Ming-Chi Kuo last summer predicted they would, Apple released a new low-cost 9... Read more
New 9.7-Inch iPad Features All Of The Fun...
Apple today updated its most popular-sized iPad, featuring a brighter 9.7-inch Retina display and best-in-class performance at its most affordable price ever, starting at $329 (US) with 32GB of... Read more
Apple Introduces iPhone 7 and iPhone 7 Plus (...
Apple today announced iPhone 7 and iPhone 7 Plus (PRODUCT)RED Special Edition in a vibrant red matte aluminum finish, in recognition of more than 10 years of partnership between Apple and (RED). This... Read more
Apple now offering Certified Refurbished 15-i...
Apple is now offering Certified Refurbished 2016 15″ Touch Bar MacBook Pros for $360-$420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
Apple Introduces Clips: A Free Innovative Way...
Apple today introduced Clips, a new app that makes it quick and fun for anyone to create expressive videos on iPhone and iPad. The app features a unique design for combining video clips, photos and... Read more
Urban Armor Gear Unveils Case For 4th Generat...
Orange County, California based Urban Armor Gear (UAG), designers of rugged, lightweight protective cases for phones, tablets and laptops, has released its latest drop-tested cases for Apple’s 4th... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Starte Dein Karriere-Abenteuer in den Hauptst...
…mehrsprachigen Teams betreust Du Kunden von bekannten globale Marken wie Apple , Mercedes, Facebook, Expedia, und vielen anderen! Funktion Du wolltest schon Read more
Starte Dein Karriere-Abenteuer in den Hauptst...
…mehrsprachigen Teams betreust Du Kunden von bekannten globale Marken wie Apple , Mercedes, Facebook, Expedia, und vielen anderen! Funktion Du wolltest schon Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.