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.

 
AAPL
$100.91
Apple Inc.
+0.38
MSFT
$44.97
Microsoft Corpora
-0.36
GOOG
$584.63
Google Inc.
-2.23

MacTech Search:
Community Search:

Software Updates via MacUpdate

Tidy Up 3.0.15.0 - Find duplicate files...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Parallels Desktop 10.0 - Run Windows app...
Parallels Desktop is simply the world's bestselling, top-rated, and most trusted solution for running Windows applications on your Mac. With Parallels Desktop for Mac, you can seamlessly run both... Read more
Apple Final Cut Pro X 10.1.3 - Professio...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Apple Compressor 4.1.3 - Adds power and...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Powerful... Read more
Chromium 36.0.1985.143 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Macgo Blu-ray Player 2.10.6.1691 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple Motion 5.1.2 - Create and customiz...
Apple Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time... Read more
A Better Finder Rename 9.39 - File, phot...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
PopChar X 6.6 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
MacUpdate Desktop 6.0.2 - Install Mac ap...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more

Latest Forum Discussions

See All

Space Colors – Tips, Tricks, Strategies,...
Hello Cadets: Want to know what we thought about this hectic space combat/roguelike? Check out our Space Colors review! Space Colors is a cool shooter/roguelike from Team Chaos. You travel from planet to planet across a huge galaxy and complete a... | Read more »
Tap Sports Baseball – Tips, Tricks, and...
Tap Sports Baseball is a pretty simple game to learn, but that doesn’t mean it’s an easy game to master, by any means. To start your batting career off well, we thought we’d give you the heads up on some handy tips and tricks. Hey Batter-Batter:... | Read more »
Tap Sports Baseball Review
Tap Sports Baseball Review By Jennifer Allen on August 20th, 2014 Our Rating: :: LET'S PLAY BALLUniversal App - Designed for iPhone and iPad Tap Sports Baseball is briefly fun but lacks some important features.   | Read more »
Frontier Heroes Review
Frontier Heroes Review By Andrew Fisher on August 20th, 2014 Our Rating: :: BLAZES NO TRAILSUniversal App - Designed for iPhone and iPad Despite awesome visuals and great music, Frontier Heroes just doesn’t quite deliver enough fun... | Read more »
Echo Prime is Now on Sale for $0.99
Echo Prime is Now on Sale for $0.99 Posted by Jessica Fisher on August 20th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Star Realms Review
Star Realms Review By Andrew Fisher on August 20th, 2014 Our Rating: :: A STAR IS BORNUniversal App - Designed for iPhone and iPad Star Realms is an excellent adaptation of an outstanding deck-builder. With great visuals and an... | Read more »
This. Is. SPRINGFIELD! War comes to The...
This. Is. SPRINGFIELD! | Read more »
One Tap RPG Review
One Tap RPG Review By Campbell Bird on August 20th, 2014 Our Rating: :: DUNGEON SLIDERUniversal App - Designed for iPhone and iPad This casual arcade game introduces some very light rpg elements into its fantasy-themed pachinko... | Read more »
Goodbye Paywall – Table Tennis Touch Eli...
Goodbye Paywall – Table Tennis Touch Eliminates In-App Purchases Posted by Jessica Fisher on August 20th, 2014 [ permalink ] | Read more »
Go to Bed – An Interview With Touchfight...
Touchfight Games is an exciting new indie studio that was co-formed between game journalist and author Nathan Meunier, artist Leonard Kenyon, and programmer Jon Kenyon. Their debut game Go To Bed will be released this fall, and with all the... | Read more »

Price Scanner via MacPrices.net

Apple now offering certified refurbished 2014...
 The Apple Store is now offering Apple Certified Refurbished 2014 MacBook Airs for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
Best Buy’s College Student Deals: $100 off Ma...
Take an additional $100 off all MacBooks and iMacs, $50 off iPad Airs and iPad minis, at Best Buy Online with their College Students Deals Savings, valid through September 6th. Anyone with a valid .... Read more
MacBook Airs on sale for $100 off MSRP, free...
B&H Photo has three 2014 MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for Laptops... Read more
Razer Taipan Mouse For Gamers And Non-Gamers...
If you’re a serious gamer on either Mac or Windows PCs, a serious gaming mouse is a necessity for first-tier performance. However, even if like me you’re not much of a gamer, there’s still a strong... Read more
15-inch 2.2GHz MacBook Pro on sale for $1899,...
Adorama has the new 15″ 2.2GHz Retina MacBook Pro on sale for $1899 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this... Read more
Mid-Size Tablet Shootout Posted: iPad mini wi...
I ‘m curious about how many iPads Apple is actually selling these days. It’s been widely rumored and anticipated that new models with A8 SoCs, 2 GB of RAM, 8 megapixel cameras, and fingerprint... Read more
The 15 Biggest iPad Air Problems And How To A...
What’s this? Fifteen “biggest” problems with the iPad Air? Does that mean there are a lot of smaller problems as well? Say it isn’t so! My old iPad 2 has manifested no hardware problems in three... Read more
TYLT Syncable-Duo, 2-in-1 USB Cable With Appl...
TYLT has introduced the Syncable-Duo, a universal cable solution for charging and syncing data to smartphones and tablets. The Syncable-Duo eliminates the need for multiple cables by incorporating... Read more
Save up to $140 off MSRP with Apple refurbish...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
2.5GHz Mac mini on sale for $549, save $50
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more

Jobs Board

Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant - Apple (United...
**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
Position Opening at *Apple* - Apple (United...
**Job Summary** Being a Business Manager at an Apple Store means you're the catalyst for businesses to discover and leverage the power, ease, and flexibility of Apple Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.