TweetFollow Us on Twitter

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

Volume Number: 25 (2009)
Issue Number: 02
Column Tag: Mac in the Shell

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

Modularizing and simplifying your code

by Edward Marczak

Introduction

We've been learning Python on the Mac and have so far covered the basics. We need to introduce a little more foundation this month. Functions-in any language-allow the author to create reusable blocks of code. This is important from a few perspectives: code reuse, the ability to refactor easily and debugging. Functions lead to building libraries, both of which are critical concepts to becoming an effective Python programmer and part of an essential foundation for creating an OS X utility or application.

Jumping In

Let's get started. Like variables, functions don't need to be defined before use. Need a function? Just define it. Similarly, functions may be defined anywhere in the source file, and in any order. Since a function defines a code block, the rules of indentation apply: choose spaces or tabs (trying to be consistent with the style you use in the rest of your code) and indent the entire function. Here's an example:

#!/usr/bin/env python
def SayHello():
  print "Hello"
SayHello()

This short program simply prints "Hello." Not too exciting, and it really could have been accomplished in one line. But it does help illustrate the basics: A function is created by using the def keyword, supplying a function name-unique to this source file-followed by parenthesis and terminated with a colon character. The lines following need to be consistently indented; the first line that lowers the indentation level ends the code block that comprises the function. The previous example is really as simple as it can get. Let's look at something a little more useful: a function definition that prints the square of a number passed into it.

def Square(number):
  print number,"squared is",number ** 2

Once defined, this function can be called as many times as you have need, individually, or in a loop:

for i in range(1,6):
  Square(i)

This will produce:

1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25

However, there's a problem with this function. What if we only want to compute the value of a square and keep it for later, rather than print it out? That's where the return keyword comes in. return sends a value back to the caller. So, our Square() function could be rewritten like this:

def Square(number):
  return number ** 2

and would need to be called like this:

x = Square(44)

or, used in-line like this:

print "The square of 78 is %d." % (Square(78))

How, you may ask, is this any better than the first version? The answer is two-fold: first, you'll typically use functions to build up more complex sections of code. The times that you have a one-line or very simple function usually involve creating a function for readability purposes. Second, by returning a value, you can store it for later, rather than display it or otherwise use it immediately.

Libraries

Let's build up a small set of useful functions, and learn some new Python skills along the way. We'll improve and expand on these functions as time goes on. Also, for now, some of this is a little more advanced than we've covered, so, just trust me for now, and this will all be covered in future columns. Here's the beginning of the code, along with the first function, and yes, it's one that you just need to trust me on for now:

Listing 1a - Wrapper for subprocess

#!/usr/bin/env python
import subprocess
def RunProc(command,env_vars=None):
  """Wrap subprocess into something reasonable.
  Args:
    command: List containing command and arguments
    env_vars: Shell environment variables that should be present for execution
  Returns:
    stdout: output of the command
  """
  proc = subprocess.Popen(command, stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE,
                          env=env_vars)
  (stdout, stderr) = proc.communicate()
  return stdout

Note that I'm also shooting for good habits, here: commented code, readable variable names and consistent, clean code style (space after comma, spaces around equal signs, and so on). The above code wraps the subprocess library calls to make it easier to run a child process. It's less than perfect right now, but we'll correct that in due time (not this column, however). Let's get on to a useful OS X-related function.

Listing 1b - GetLocalUsers function.

def GetLocalUsers():
  """Fills a dictionary with all system users
     Args:
       None
     Returns:
       Dictionary, filled with uid/username pairs       
  """
  command = ['/usr/bin/dscl','.','list','/Users']
  output = RunProc(command)
  userList = output.split('\n')
  userDict = {}
  for userName in userList:
    if userName is not '':
      command = ['dscl','.','read','/Users/'+userName,'uid']
      output = RunProc(command)
      uidLocation = output.split(':')
      uid = uidLocation[2].strip()
      userDict[uid] = userName
  return userDict

The GetSystemUsers() function will fill a dictionary with uid (the key) and the short name that it is assigned to. It takes no parameters, and can be called like this:

userDict = GetLocalUsers()

To extract information, simply request the value using the uid as the key:

print "User ID 74 is %s." % (userDict[74])

(It's _mysql, if you're curious and not typing in code while reading this).

There are some new concepts in Listing 1b. The split string function (line 10 of listing 1b), creates a list, each element created by the boundary of the character argument. For example, the string 'This is a string' when split using a space (string.split(' ')) becomes the following list:

['This', 'is', 'a', 'string']

Any character can be used to provide the boundary marker.

This function is nice, but could certainly be improved. What if we only wanted standard users, and not all of the system users? That's where function arguments come in. Like passing arguments into a command line application, functions can accept arguments, too. Alter our function to do so:

def GetLocalUsers(incSystemUsers):

This will allow us to pass a value into the function. Great, but we need to do something with it. Let's expect this to be a Boolean True or False: if True, include the system accounts, if False, do not. (I'm classifying accounts based on uid-only system accounts have a uid of less than 500). Update the code portion of the function to utilize this new variable (the first ten lines are the same as before-I just wanted to give some context):

command = ['/usr/bin/dscl','.','list','/Users']
  output = RunProc(command)
  userList = output.split('\n')
  userDict = {}
  for userName in userList:
    if userName is not '':
      command = ['dscl','.','read','/Users/'+userName,'uid']
      output = RunProc(command)
      uidLocation = output.split(':')
      uid = uidLocation[2].strip()
      if incSystemUsers:
        userDict[int(uid)] = userName
      else:
        if int(uid) > 499:
          userDict[int(uid)] = userName
  return userDict

We use a simple if/else statement to determine if the user should be included in the dictionary that is returned. We should update our comments about this in the function, too.

Now, we have to include a value when we call the function. Like this:

userDict = GetLocalUsers(False)

This is an improvement to this function. Frankly, though, more often than not, you probably do only want standard users.

Default Arguments

Python functions understand default arguments. Change the function definition to read:

def GetLocalUsers(incSystemUsers=False):

Now, if we don't tell GetLocalUsers how to behave, by not including an argument, it will assign False and continue on. So now, we can once again call the function with no arguments:

userDict = GetLocalUsers()

This is equivalent to calling GetLocalUsers(False). We really only have to pass in a value of True if we wish to override the default behavior. It is possible to combine default and non-default arguments in a single function definition. For example:

def Mount(path, method="afp", mount_point="/Volumes", shadow=False):

This fictional function, Mount(), has four parameters, only one which you are required to provide when called: path. The remainder use the defaults provided. Therefore, this function can be called in the following ways:

Mount("serv.example.com/share")
Mount("serv.example.com/share", "nfs")
Mount("serv.example.com/share","/tmp/mnt")

These examples use simple positional arguments: you need to know the position of the argument and pass in the values in the correct sequence. When creating a function definition, default arguments should be grouped at the end of the list. This allows them to be omitted.

Python also supports specifying the argument that is being passed in. This allows us to call the function like this:

Mount("serv.example.com/data.dmg","http",shadow=True)

Note that we're passing in the value for shadow in the third position, where we would typically specify mount_point. This is an excellent way to pass parameters into a function, as it makes it exceptionally obvious what is taking place. Which of the following code is easier to read?

ConvertFile('rawdata.txt','sales.csv','csv',True)

or:

ConvertFile(infile='rawdata.txt', outfile='sales.csv',
            format='csv',Totals=True)

Hmmmm? Using named arguments is a good habit to get into.

Variable Length Argument Lists

As a final discourse in functions, what if you don't know how many parameters a function may receive, or, has a number that may change from call to call? Python supports variable length argument lists. Two decorators designate how these values are received. If a single asterisk is used, the values are received into a tuple, receiving any excess positional parameters, defaulting to an empty tuple. If a double asterisk is used, it is initialized to a new dictionary receiving any excess keyword arguments, defaulting to a new empty dictionary. Let's see how this works. Here's a short, sample function that accepts any number of arguments:

def SomeFunction(*args):
  print args

It can be called like this

SomeFunction(24, 'blah', 'cold', 88, [22,34,66],'starch')

Note that arguments do not need to be of the same type. Pass in strings, integers, or even lists and dictionaries. The same goes for the double-asterisk decorator, with one exception: since you're creating a dictionary, you need to pass the key and the value.

Changing the function definition to:

def SomeFunction(**args):

allows the function to be called like this:

SomeFunction(number=24, some_string='blah',
             a_list = [22,34,66], string2='starch')

In our case, this will cause the function to output:

{'a_list': [22, 34, 66], 'some_string': 'blah', 'string2': 'starch', 'number': 24}

Finally, realize that you can mix variable arguments along with standard and named arguments. In the examples used above, since we only specified a variable argument, passing in an argument is completely optional.

In Conclusion

I was hoping to rip through functions and talk about modules in this column, but then I started writing, and realized the scope of the topic. I want to do justice to both functions and modules, as they're both critical foundational subjects. Next month, we'll build on the functions introduced this month and dig into modules.

Media of the month: I'm not going to call out any one particular 'thing,' but will give this directive: Find something new. If you've only been exposed to the Mac, go pick up a book (or find a web page) about Windows, FreeBSD or Linux. Compare to OS X. If you're a MySQL person, download and explore PostgreSQL, or learn the command-line interface to SQLite. You get the idea-stretch your boundaries.


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

OmniOutliner Pro 4.6 - Pro version of th...
OmniOutliner Pro is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually designed to help you think. It's... Read more
Alfred 3.1 - Quick launcher for apps and...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
OmniOutliner 4.6 - Organize your ideas,...
OmniOutliner is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually designed to help you think. It's... Read more
Default Folder X 5.0.6 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Adobe Creative Cloud 3.8.0.310 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
MYStuff Pro 2.0.25 - Create inventories...
MYStuff Pro is the most flexible way to create detail-rich inventories for your home or small business. Add items to MYStuff by dragging and dropping existing information, uploading new images, or... Read more
Viber 6.2.0 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more
Data Rescue 4.2.3 - Powerful hard drive...
Use Data Rescue to recover: crashed, corrupted or non-mounting hard drive deleted, damaged, or lost files reformatted or erased hard drive One powerful new feature found in Data Rescue 4 is... Read more
Microsoft Remote Desktop 8.0.34 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more
Microsoft Remote Desktop 8.0.34 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more

Find out the story behind League of Ange...
If you’re looking for a new thrilling MMORPG to play with your friends then you’ll be excited to hear that there is a sequel to one of the most well known titles in the genre – namely League of Angels 2. With a brand new 3D engine offering... | Read more »
Naruto Shippuden: Ultimate Ninja Blazing...
I'm not sure if it's possible to say you are an anime fan but also never have seen one episode of Naruto. If it is, then I resemble that remark, and if not, I offer a hearty apology. [Read more] | Read more »
5 mobile games that let you explore spac...
No Man's Sky hasn't exactly turned out to be everything it was promised. Though its core concept of exploring an unimaginably vast universe of different planets is an intriguing one, the execution has left many PS4 and PC gamers feeling like they... | Read more »
Mummy madness in new action game Tomb He...
Hot on the tail of Bump Hero, ZPlay is giving gamers another reason to get screen bashing with a brand new release. Tomb Heroes is a challenging action game in which you battle enemies in various tombs around the world. You can select from nine... | Read more »
Siralim 2 (RPG / Roguelike) (Games)
Siralim 2 (RPG / Roguelike) 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Siralim 2 is an old-school monster catching RPG. Summon and customize hundreds of creatures to fight for you as... | Read more »
Clean Text (Productivity)
Clean Text 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Gemini - A Journey of Two Stars (Games)
Gemini - A Journey of Two Stars 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: *** SPECIAL LAUNCH SALE: $2.99 (25% off) *** "A mesmerizing and unexpectedly emotional journey." -- Los... | Read more »
How to get four NFL superstars for your...
Even though you're probably well on your way to building a top notch squad for the new season in Madden NFL Mobile, let's say you could beef it up by adding Rob Gronkowski, Antonio Brown, Von Miller, and Todd Gurley to your roster. That's... | Read more »
Cartoon Network Superstar Soccer: Goal!!...
Cartoon Network Superstar Soccer: Goal!!! – Multiplayer Sports Game Starring Your Favorite Characters 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Become a soccer superstar with your... | Read more »
NFL Huddle: What's new in Topps NFL...
Can you smell that? It's the scent of pigskin in the air, which either means that cliches be damned, pigs are flying in your neck of the woods, or the new NFL season is right around the corner. [Read more] | Read more »

Price Scanner via MacPrices.net

RESCUECOM 2016 Semi-Annual Computer Reliabili...
The beginning of a new school year is upon us again, in which students and parents have some very important choices to make, often including the purchase of a computer or tablet. Whether you are... Read more
VRS Design Damda Glide Series iPhone 7 and 7...
What makes the Damda Glide Series for the iPhone 7 and iPhone 7 Plus special? Case maker VRS Design says its Damda Glide Series is the first mobile case to incorporate a semi-automatic mechanism for... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $350 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $829... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Save $120 with Apple refurbished Time Capsule...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off (not... Read more
9-inch 32GB iPad Pros on sale for $70 off MSR...
B&H Photo has 9″ 32GB WiFi Apple iPad Pros on sale for $70 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 32GB WiFi iPad Pro: $529 $70 off MSRP - 9... Read more
Mac minis on sale for up to $140 off MSRP
Adorama has Mac minis on sale for up to $100 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP Amazon has the... Read more
Back To School with OtterBox Essentials
Back to school means back to an environment that is tough on tech. OtterBox has the back to school essentials you need to keep tech safe from drops, bumps, scratches and hallway havoc. Check out the... Read more
VRS Design Releases New iPhone 7 Plus Case Li...
With a device as large and costly as the iPhone 7 Plus, it is primal instinct to protect it from potential damage. According to a study by SquareTrade in 2012, iPhone damages cost Americans roughly $... Read more

Jobs Board

*Apple* Solutions Consultant - Apple (United...
Apple Solutions ConsultantJob Number: 51218534Pleasant Hill, California, United StatesPosted: Aug. 18, 2016Weekly Hours: 40.00Job SummaryAs an Apple Solutions Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 51218354 Fredericksburg, Virginia, United States Posted: Aug. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* /Mac Support Engineer - GFI Digital,...
FI Digital, Inc. is currently seeking candidates for a full time Apple Support Engineer to add to our Maryland Heights, Missouri IT team. Candidates must be dynamic Read more
*Apple* Solutions Consultant - Apple (United...
Apple Solutions ConsultantJob Number: 51218534Pleasant Hill, California, United StatesPosted: Aug. 18, 2016Weekly Hours: 40.00Job SummaryAs an Apple Solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.