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.

 
AAPL
$98.44
Apple Inc.
+0.06
MSFT
$43.64
Microsoft Corpora
-0.25
GOOG
$586.77
Google Inc.
+1.16

MacTech Search:
Community Search:

Software Updates via MacUpdate

Mellel 3.3.6 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
LibreOffice 4.3.0.4 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Freeway Pro 7.0 - Drag-and-drop Web desi...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With it's user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Drive Genius 3.2.4 - Powerful system uti...
Drive Genius is an OS X utility designed to provide unsurpassed storage management. Featuring an easy-to-use interface, Drive Genius is packed with powerful tools such as a drive optimizer, a... Read more
Vitamin-R 2.15 - Personal productivity t...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Toast Titanium 12.0 - The ultimate media...
Toast Titanium goes way beyond the very basic burning in the Mac OS and iLife software, and sets the standard for burning CDs, DVDs, and now Blu-ray discs on the Mac. Create superior sounding audio... Read more
OS X Yosemite Wallpaper 1.0 - Desktop im...
OS X Yosemite Wallpaper is the gorgeous new background image for Apple's upcoming OS X 10.10 Yosemite. This wallpaper is available for all screen resolutions with a source file that measures 5,418... Read more
Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more

Latest Forum Discussions

See All

NFL Fantasy Football App Redesigned Ahea...
NFL Fantasy Football App Redesigned Ahead of Upcoming 2014 Season Posted by Ellis Spice on July 30th, 2014 [ permalink ] | Read more »
Matter Review
Matter Review By Jennifer Allen on July 30th, 2014 Our Rating: :: ORIGINAL PHOTO MANIPULATIONUniversal App - Designed for iPhone and iPad Matter lets you add geometric 3d shapes to your images and manipulate things so each image... | Read more »
Note Review
Note Review By Jennifer Allen on July 29th, 2014 Our Rating: :: TOO SIMPLEiPhone App - Designed for the iPhone, compatible with the iPad Note is a note taking app that’s a little too short on features to be worth its asking price... | Read more »
Chainsaw Warrior Goes on Sale & Ther...
Chainsaw Warrior Goes on Sale & There’s a Chance to Win a Copy of the Original Board Game Posted by Jennifer Allen on July 29th, 2014 [ permalink | Read more »
It Came From Canada: Tiny Tower Vegas
If you go to a casino, you might make a lot of money. If you run a casino, you’re guaranteed to make a lot of money. The choice seems pretty obvious. So while waiting for your shady real estate deals to move forward, get prepared with Tiny Tower... | Read more »
Z Hunter Review
Z Hunter Review By Lee Hamlet on July 29th, 2014 Our Rating: :: RIGHT ON TARGETUniversal App - Designed for iPhone and iPad While it might not necessarily break new ground, Z Hunter has enough tricks up its sleeve to ensure that... | Read more »
Huge Update Comes To Duet, Adding 48 New...
Huge Update Comes To Duet, Adding 48 New Stages Posted by Jennifer Allen on July 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Sharknado: The Video Game Available Now....
Sharknado: The Video Game Available Now. Seriously. Posted by Rob Rich on July 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Frog Orbs 2 Review
Frog Orbs 2 Review By Nadia Oxford on July 29th, 2014 Our Rating: :: THIS MAGIC IS A TAD MONOTONOUS Universal App - Designed for iPhone and iPad Frog Orbs 2 is repetitive, but younger players should enjoy it nonetheless.   | Read more »
Puzzix Review
Puzzix Review By Jennifer Allen on July 29th, 2014 Our Rating: :: NICE IDEAUniversal App - Designed for iPhone and iPad A little like Tetris, Puzzix is all about piecing together blocks and watching them vanish. It could do with... | Read more »

Price Scanner via MacPrices.net

WaterField Unveils 15″ Outback Solo & 13″...
Hard on the heels of Apple’s refreshed MacBook Pro Retina laptops announcement, WaterField Designs has unveiled a 15-inch version of the waxed-canvas and leather Outback Solo and a 13-inch version of... Read more
New Roxio Toast 12 Delivers Digital Media Pow...
Roxio Toast 12 is a hub for sharing digital media to virtually any platform or device. has introduced two new additions to its Roxio Toast product family – Roxio Toast 12 Titanium and Roxio Toast 12... Read more
The lowest prices on leftover Retina MacBook...
Best Buy has dropped prices on leftover 13″ and 15″ Retina MacBook Pros by up to $300 off original MSRP on their online store for a limited time. Choose free local store pickup (if available) or free... Read more
Apple Updates MacBook Pro with Retina Display...
Apple today updated its MacBook Pro with Retina display with faster processors and double the amount of memory in both entry-level configurations. MacBook Pro with Retina display features a Retina... Read more
Up to $250 price drop on leftover 15-inch Mac...
B&H Photo has dropped prices on 2013 15″ Retina MacBook Pros by as much as $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.3GHz Retina MacBook Pro: $2349... Read more
Updated MacBook Pro Price Trackers
We’ve updated our MacBook Pro Price Trackers with the latest information on prices, bundles, and availability on the new 2014 models from Apple’s authorized internet/catalog resellers as well as... Read more
Apple updates MacBook Pros with slightly fast...
Apple updated 13″ and 15″ Retina MacBook Pros today with slightly faster Haswell processors. 13″ models now ship with 8GB of RAM standard, while 15″ MacBook Pros ship with 16GB across the board. Most... Read more
Apple drops price on 13″ 2.5GHz MacBook Pro b...
The Apple Store has dropped their price for the 13″ 2.5GHz MacBook Pro by $100 to $1099 including free shipping. Read more
Apple drops prices on refurbished 2013 MacBoo...
The Apple Store has dropped prices on Apple Certified Refurbished 13″ and 15″ 2013 MacBook Pros, with model now available starting at $929. Apple’s one-year warranty is standard, and shipping is free... Read more
iOS 8 and OS X 10.10 To Support DuckDuckGo As...
Writing for Quartz, Dan Frommer reports that Apple’s forthcoming iOS 8 and OS X 10.10 operating systems version updates will allow users to select DuckDuckGo as their default search engine. He notes... Read more

Jobs Board

Sr Software Lead Engineer, *Apple* Online S...
Sr Software Lead Engineer, Apple Online Store Publishing Systems Keywords: Company: Apple Job Code: E3PCAK8MgYYkw Location (City or ZIP): Santa Clara Status: Full Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.