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

Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
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 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Cloud 3.3.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
Cyberduck 4.6.5 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Firefox 36.0 - Fast, safe Web browser. (...
Firefox for Mac 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... Read more
Thunderbird 31.5.0 - Email client from M...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
VOX 2.4 - Music player that supports man...
VoxIt just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features & support for all audio formats you should ever need... Read more

Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Who Wore it Best? The Counting Dead vs....
Like it or not, the “clicker” genre, popularized by cute distractions like Candy Box and Cookie Clicker, seems like it’s here to stay. So Who Wore it Best? takes a look at two recent examples: The Counting Dead and AdVenture Capitalist. | Read more »
Card Crawl, the Mini Deck Building Game,...
Card Crawl, the Mini Deck Building Game, is Coming Soon Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Tinytouchtales and Mexer have announced their new game, | Read more »
Witness an all new puzzle mechanic in Bl...
Well, BlastBall MAX is not one of those games and is bucking trends such as timers, elements of randomness, and tacked-on mechanics in favor of pure puzzle gameplay. When you first boot up the game you’ll see a grid made up of squares that are each... | Read more »
This Princess Has a Dragon and She isn’t...
This Princess Has a Dragon and She isn’t Afraid to Useit. | Read more »
Mecha Showdown Review
Mecha Showdown Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IN A SPINUniversal App - Designed for iPhone and iPad Mecha Showdown replaces traditional buttons with a slot machine mechanic in this robot fighting game,... | Read more »
Reliance Games and Dreamworks Unveil Rea...
Reliance Games and Dreamworks Unveil Real Steel Champions Posted by Ellis Spice on February 27th, 2015 [ permalink ] Reliance Games and Dreamworks have announced that a third game in | Read more »
Sum Idea Review
Sum Idea Review By Jennifer Allen on February 27th, 2015 Our Rating: :: TAXING NUMBERSUniversal App - Designed for iPhone and iPad Sum Idea is a fairly charming but taxing puzzle game.   | Read more »
A New Badland Update Brings Daydream Lev...
A New Badland Update Brings Daydream Levels to Co-Op Posted by Ellis Spice on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Slashing Demons Review
Slashing Demons Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IT'S A LONG WAY TO THE TOPUniversal App - Designed for iPhone and iPad Slashing Demons lacks the depth or scope to take it beyond the point of being just... | Read more »

Price Scanner via MacPrices.net

Save up to $600 with Apple refurbished Mac Pr...
The Apple Store is offering Apple Certified Refurbished Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more
iMacs on sale for up to $205 off MSRP
B&H Photo has 21″ and 27″ iMacs on sale for up to $205 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $1029 $70 off - 21″ 2.7GHz iMac: $1199 $100 off - 21″ 2.9GHz... Read more
Apple Takes 89 Percent Share of Global Smartp...
According to the latest research from Strategy Analytics, global smartphone operating profit reached US$21 billion in Q4 2014. The Android operating system captured a record-low 11 percent global... Read more
New Travel Health App “My Travel Health” iOS...
Rochester, Minnesota based Travel Health and Wellness LLC has announced that its new iOS app help safeguard the user’s health when traveling abroad — “My Travel Health” is now available on the Apple... Read more
Sale! MacBook Airs for up to $115 off MSRP
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more

Jobs Board

Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
*Apple* Pay - Site Reliability Engineer - 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 - Retail Sales...
**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 - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.