TweetFollow Us on Twitter

Mac in the Shell: Reading and Writing plist files with Python

Volume Number: 25
Issue Number: 09
Column Tag: Mac in the Shell

Mac in the Shell: Reading and Writing plist files with Python

Tame those pesky plists

by Edwarcd Marczak

Welcome

Property list files, also known as 'plists,' are pervasive in OS X. This article teaches you the basic inner-workings of the plist format, system level methods of working with plist files and how to interact with these files using Python under OS X.

Anatomy

Plist files are structured XML (eXtensible Markup Language) files and easily understandable. Essentially, a plist file is a way to store standard types of data. By "standard," I mean string, integer, Boolean and so on, although there are ways to store arbitrary data as well. A plist file can easily be read into and written out from an NSDictionary object. Thanks to PyObj-C, an NSDictionary can be mapped onto and manipulated with a Python-based dictionary object.

Given the following dictionary:

{
    color:'blue',
    count:15,
    style:'fruit'
}

the plist in Listing 1 would be created

Listing 1-example plist file

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
   <key>color</key>
   <string>blue</string>
   <key>count</key>
   <integer>15</integer>
   <key>style</key>
   <string>fruit</string>
</dict>
</plist>

Let's take a closer look at this plist. The header declares this file as an XML

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

Ultimately, this header isn't up to you. In this article, you'll see that Apple's Cocoa APIs will properly generate this upon writing a plist. For more information about XML, see the specification page as http://xml.org, or the Wikipedia entry at http://en.wikipedia.org/wiki/Xml.

The plist tag wraps the entire file:

<plist version="1.0">

Again, Apple's APIs will write this out as appropriate. Next, we find a dictionary tag:

<dict>

As I mentioned earlier, the structure we wrote out was a dictionary. In fact, that's all you'll ever really do with plist files: read a plist into a dictionary or create one from scratch, and then let Apple's APIs write it out.

Wrapped in the dictionary are its values:

<key>color</key>
<string>blue</string>
<key>count</key>
<integer>15</integer>
<key>style</key>
<string>fruit</string>
Following this, the tags are closed and the file ends:
</dict>
</plist>

Each tag should lead to a new level of indentation. It's easy to see the structure here. Best of all, it's easily human-readable.

However, beginning with OS X 10.5, the bulk of plist files found on the system are stored in a binary format, not plain text. While this does have the effect of using less space on disk and faster load times, it takes the human-readable part out of the picture. Of course, there are ways to deal with that.

System Tools

There are several ways to work with plist files, both graphically and from the command line. Apple's Property List Editor is installed as part of the free developer tools suite (Xcode et al). In a standard install, it is found at /Developer/Applications/Utilities/Property List Editor.app. This is the easiest way to visualize a plist. It's also useful for creating a plist from scratch. Property List Editor can also edit entries in a plist file.


Figure 1-Property List Editor.app displaying the hierarchy of a plist file.

While Property List Editor is fine for one-off plist work, it doesn't really scale too well. That it doesn't have a dictionary to use with AppleScript is just one example. What if you need to modify a plist on thousands of machines? (Or even 15 machines-it's a pain to walk around to each machine and potentially interrupt people's work. You may even want to update them after hours, while you're home). Once again, it's scripting to the rescue.

There are several utilities for standard shell scripting or ad-hoc use. plutil, defaults and PlistBuddy all have different purposes and capabilities.

plutil is the most basic and utilitarian of the three. plutil, the plist utility, converts plist files between text (xml) and binary formats and can also verify the structure of a plist. An example is in order. If you want to view the contents of a binary plist-com.apple.nat.plist, for example-but don't care to open it in Property List Editor you can run this:

plutil -convert xml1 -o - /Library/Preferences/com.apple.nat.plist

(This makes a very nice alias: alias viewplist="plutil -convert xml1 -o - $1". Keep that in your .bash_profile). Running this command tells plutil to convert the plist to text ("xml1") and send the output ("-o") to standard out. You could certainly write the output to another file on disk if you choose.

plutil can also lint a file; that is, check it for consistency and basic errors. What it cannot do is verify that your key-names and data are correct. Running a lint check is as simple as passing in the -lint switch:

$ plutil -lint /Library/Preferences/com.apple.loginwindow.plist 
/Library/Preferences/com.apple.loginwindow.plist: OK

If the lint process encounters an error (or errors, perhaps), you're told the error and on which line:

$ plutil -lint someplist 
someplist: Encountered unknown tag stringblue</string on line 6

The defaults command gives you access to the user defaults system. The "user defaults system" is a fancy way of saying "preferences," which, you'll probably recognize as data stored in a plist file. The name is derived from the Cocoa API that performs the same task: NSUserDefaults. The defaults utility allows for reading and writing individual keys and their data to and from a plist file, reading a plist in whole and more.

Perhaps the simplest use of the defaults command is reading an entire plist file. This is equivalent to the plutil command given earlier:

$ defaults read /Library/Preferences/com.apple.nat
{
    NatPortMapDisabled = 0;
}

The defaults command reads plist files of either xml or binary. However, it will only write a plist out in the binary variety. It will even go so far as to convert an xml plist into binary if used to update a value in that plist. Do note that the target plist is specified without the .plist extension.

The defaults command, however, is not exactly a general-purpose plist utility like plutil or Propery List Editor.app. As mentioned, it works within the bounds of the user defaults system. The upshot of this is that it expects plists to reside in specific places: one of the Library/Preferences directories on the system. Do not rely on the defaults command to read and write arbitrary plists. (In 10.5 and 10.6, accessing arbitrary plist files is possible, however, that functionality is said to be going away. Plus, you're reading this article and will be learning better ways of handling this). One other small problem with defaults: it's virtually impossible to work with values in nested dictionaries. Which brings us to PlistBuddy.

PlistBuddy started off as a utility that was only found embedded into packages for Apple updates. Clearly, Apple realized they needed a utility like this and developed it for their own use. As of Leopard, though, it's a real part of the OS: it is found at /usr/libexec/PlistBuddy and even has a man page. While the defaults command can handle most tasks, PlistBuddy excels at editing keys and values in a nested dictionary.

Let's imagine our example plist looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
   <key>color</key>
   <string>blue</string>
   <key>count</key>
   <integer>15</integer>
   <key>cust_info</key>
   <dict>
      <key>pid</key>
      <string>98234573</string>
      <key>uid</key>
      <string>348576</string>
   </dict>
   <key>style</key>
   <string>fruit</string>
</dict>
</plist>

Notice that the key, "cust_info" is a dictionary, rather than a simple, single value. PlistBuddy can easily update the values in this nested dictionary. PlistBuddy can work interactively, which I will not cover, but can also pass in all commands using the "-c" switch. To set the value of a key, you need the path to the key and the set command. The path to the key starts with a colon (":") and uses a colon as the separator for each level in the hierarchy. Here's how to change ("set") the value of the existing "pid" key to 94758476, in the plist, "com.mactech.example.plist":

/usr/libexec/PlistBuddy -c "Set :cust_info:pid 94758476" ./com.mactech.example.plist

(This is running the command in the same directory as the target plist. Otherwise, you'd need to specify the full path to the plist to edit). See the PlistBuddy man page (note the capitalization!) for more information on the utility. PlistBuddy is capable of much, much more, including copying values and merging plist files.

Accessing plists Via Python

From time to time, as a system administrator, you'll find yourself in a position where you'd like a script to store its own preferences. Or, simply have a script analyze a plist and act on the contents in some manner. In many cases, bash scripting that uses the commands already presented (plutil, PlistBuddy and, particularly, defaults) will be perfectly acceptable. However, for anything with a little more complexity, you may already be scripting in Python (or perl, or Ruby, etc.). Since Mac in the Shell has been focusing on Python for the last several columns, we'll use it here as well.

Python, with PyObj-C, makes this trivial. More interestingly, you get the best of both worlds: Apple's APIs along with Python's ease of use and the speed of the edit and run cycle (skipping the compile step of C-based languages). To see this in action, let's start with nearly the most simple example possible. Listing 2 contains write_plist.py, which demonstrates creating a dictionary that gets written to a plist.

Listing 2-write_plist.py

#!/usr/bin/python2.5
from Foundation import NSMutableDictionary
my_dict = NSMutableDictionary.dictionary()
my_dict['color'] = 'blue'
my_dict['count'] = 15
my_dict['style'] = 'fruit'
success = my_dict.writeToFile_atomically_('com.mactech.example.plist', 1)
if not success:
  print "plist failed to write!"
  sys.exit(1)

Upon running this program, com.mactech.example.plist will be created in the same working directory as the program itself. The plist file will match the output that is shown in Listing 1. Let's examine this line-by-line to see how it works.

The very first line-#!/usr/bin/python2.5-is a good reminder that Python version 2.5 or higher is required for PyObj-C integration. This will not work on Tiger systems out of the box.

from Foundation import NSMutableDictionary

This import is responsible for all of the magic here. While we could import all of Foundation, we'll just import the portion we need: NSMutableDictionary.

my_dict = NSMutableDictionary.dictionary()
-

Typically, creating a dictionary in Python would use curly braces, like this:

new_dict = {}

or, you can even fill it on creation:

new_dict = {'color':'blue', 'count':15, 'style':'fruit'}

However, we need to create a real Cocoa NSMutableDictionary object, so that's what we've done. Nicely, we can no go on and treat that just like a Python dictionary:

my_dict['color'] = 'blue'
my_dict['count'] = 15
my_dict['style'] = 'fruit'

You can use the Cocoa API for adding entries to a dictionary as well:

my_dict.setValue_forKey_('stop', 'state')

This would set the key 'state' to store the value 'stop', and add the following to the plist once written out:

<key>state</key>
<string>stop</string>

But, really... if you're using Python, take advantage of it where you can! (I suggest using the Python method). You will need to use the Cocoa API to write the dictionary out to disk as a plist file:

success = my_dict.writeToFile_atomically_('com.mactech.example.plist', 1)

The Cocoa writeToFile:atomically: method of NSDictionary (and, by extension, NSMutableDictionary) writes a property list representation of the contents of the dictionary to the path given.

if not success:
  print "plist failed to write!"
  sys.exit(1)

This final conditional tests to see if the writeToFile:atomically: method returned a True ("success") or False ("failure") value. While not strictly necessary for this program to run, checking these values is a good habit to get into.

Python Ease

Just as a reminder, once you create the NSMutableDictionary, you can use standard Python mechanisms to manipulate and traverse it. Adding a key with a dictionary as its value is as simpe as you'd expect. Just create the dictionary and then assign it to the parent dictionary. For example, to recreate the com.mactech.example.plist shown earlier, we would add the following to our program, after creating the initial dictionary:

sub_dict = {}
sub_dict['uid'] = '348576'
sub_dict['pid'] = '98234573'
my_dict['cust_info'] = sub_dict

Also, as shown earlier, you can also use all of the Cocoa APIs available to you to manipulate the dictionary as well. The style you choose may be situation dependent. Some situations may call for using the Cocoa-way, while others may favor more Pythonic writing. When working with any Cocoa API, though, as always, you'll want to keep the documentation handy.

Use It or Lose It

This was an incredibly fun article to write. The topic is incredibly practical for everyday use. The plist format is pervasive throughout OS X. Every technical person should have a familiarity with it, and System Admins should be even more deeply involved. While many cases can simply be solved with a single command-line call to defaults or PlistBuddy, anything with deeper involvement should use a scripting language like Python. The nice thing about the scripting solution is that once you build up your library of routines, they're written and ready for re-use. Reading about it here only gets you so far. Go write a script and run it on a test system so you're ready for the real thing when the opportunity arrives.

Media of the month: Kick it old-school with vintage computer brochures and manuals at http://assemblyman-eph.blogspot.com/2009/04/vintage-computer-brochures.html. Full PDFs of how it used to be. I remember when taking one of my first computer courses, the teacher launching into the history of computing. Naturally, I just wanted to get into sitting at a computer and coding. But now, perhaps more than ever before, it's really useful to be able to frame our current experience with that which it was built on and evolved from.

Next month, we'll be covering Snow Leopard related topics! It'll be two issues before we get back to Python and scripting in general. Until then, keep practicing.

References

"About Property Lists": https://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/PropertyLists/AboutPropertyLists/AboutPropertyLists.html#/apple_ref/doc/uid/20001010-46719

"Understanding XML Property Lists": http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/PropertyLists/UnderstandXMLPlist/UnderstandXMLPlist.html#/apple_ref/doc/uid/10000048i-CH6-SW1

"Introduction to Property List Programming Topics for Core Foundation": http://developer.apple.com/iphone/library/documentation/CoreFoundation/Conceptual/CFPropertyLists/CFPropertyLists.html

"Introduction to User Defaults": http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/PropertyLists/UnderstandXMLPlist/UnderstandXMLPlist.html#/apple_ref/doc/uid/10000048i-CH6-SW1


Ed Marczak is the Executive Editor of MacTech Magazine. He has written for MacTech since 2004.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Paragraphs 1.0.1 - Writing tool just for...
Paragraphs is an app just for writers. It was built for one thing and one thing only: writing. It gives you everything you need to create brilliant prose and does away with the rest. Everything in... Read more
BlueStacks App Player 0.9.21 - Run Andro...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Version 0.9.21: Note: Now requires OS X 10.8 or later running on a 64-bit Intel processor. Initial stable... Read more
Apple iTunes 12.2 - Play Apple Music...
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
Apple Security Update 2015-005 - For OS...
Apple Security Update 2015-005 is recommended for all users and improves the security of OS X. For detailed information about the security content of this update, please visit: http://support.apple.... Read more
Apple HP Printer Drivers 3.1 - For OS X...
Apple HP Printer Drivers includes the latest HP printing and scanning software for OS X Lion or later. For information about supported printer models, see this page. Version 3.1: The latest printing... Read more
Epson Printer Drivers 3.1 - For OS X 10....
Epson Printer Drivers installs the latest software for your EPSON printer or scanner for OS X Yosemite, OS X Mavericks, OS X Mountain Lion, and OS X Lion. For more information about printing and... Read more
Xcode 6.4 - Integrated development envir...
Xcode provides everything developers need to create great applications for Mac, iPhone, and iPad. Xcode brings user interface design, coding, testing, and debugging into a united workflow. The Xcode... Read more
OS X Yosemite 10.10.4 - Apple's lat...
OS X Yosemite is Apple's newest operating system for Mac. An elegant design that feels entirely fresh, yet inherently familiar. The apps you use every day, enhanced with new features. And a... Read more
Dash 3.0.2 - Instant search and offline...
Dash is an API Documentation Browser and Code Snippet Manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
FontExplorer X Pro 5.0 - Font management...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more

Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Hands-On With Raceline CC
Set for release soon, Rebellion’s motorbike racing game, Raceline CC certainly looks stylish. But how does it play? I got my hands on a preview build to answer exactly that. | Read more »
Siegefall - Tips, Tricks, and Strategies...
So, you fancy establishing a base and ruling the world again. Siegefall is a convenient place to do that, but how about some great tips and tricks on how best to go about it? Here are a few ideas on how to get ahead as a beginner to this medieval... | Read more »
The WWE Comes to Racing Rivals - Because...
Racing Rivals is a racing game that's all about, well, rivalry. And who knows rivalry better than WWE superstars (shhhh, that was rhetorical)? [Read more] | Read more »
Hey, Who Put Apple Music in My SoundHoun...
One of the App Store's popular music discovery sources - SoundHound - has already been updated to include Apple's own music discovery source - Apple Music. That was fast! [Read more] | Read more »
Arcane Legends has a New Expansion Calle...
Arcane Legends has been going strong since it debuted at the tail end of 2012. So well, in fact, that it's already up to its sixth expansion. [Read more] | Read more »
Vector 2 is Officially a Thing and it...
Vector is a pretty cool parkour-driven runner that's gotten a pretty decent following since it first came out - although personally I think more people could stand to show it some love. Anyway, Nekki has announced that a sequel isofficially on its... | Read more »
Get Ready to Trucksform and Roll Out (an...
It looks like NuOxygen is bringing the truck-transforming racer Trucksform (get it?) to iOS in a couple of weeks. Although really it's more of an auto-driver than a racer. But still, transforming trucks! [Read more] | Read more »
This Week at 148Apps:June 22-26, 2015
June's Summer Journey Continues With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice,... | Read more »
LEGO® Minifigures Online (Games)
LEGO® Minifigures Online 1.0.1 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0.1 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Logo Pop Free Vector Logo Design App For OS X...
128bit Technologies has released of Logo Pop Free 1.2 for Mac OS X, a vector based, full-fledged, logo design app available exclusively on the Mac App Store for the agreeable price of absolutely free... Read more
21-inch 1.4GHz iMac on sale for $999, save $1...
B&H Photo has new 21″ 1.4GHz iMac on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Best Buy has the 21″ 1.4GHz iMac on sale for $999.99 on their... Read more
16GB iPad mini 3 on sale for $339, save $60
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $339 including free shipping plus NY tax only. Their price is $60 off MSRP. Read more
Save up to $40 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $40 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $489 $10 off - 64GB iPad Air 2 WiFi: $559 $40 off - 128GB iPad Air... Read more
Apple Releases OS X 10.10.4 With WIFi Fix, iO...
On Tuesday, Apple released final versions of OS X 10.10.4 and iOS 8.4, as well as updates for the Safari browser for OS X Yosemite, Mavericks, and Mountain Lion. The OS X 10.10.4 update focuses on... Read more
Dual-Band High-Gain Antennas for Home Wi-Fi N...
Linksys has announced what it claims are the first dual-band, omni-directional high-gain antennas for the consumer market. The new Linksys high-gain antennas available in a 2- and 4-pack (WRT004ANT... Read more
Apple refurbished 2014 15-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have... Read more
Clearance 2014 MacBook Airs available for up...
Adorama has 2014 MacBook Airs on sale for up to $301 off original MSRP including NY + NJ sales tax and free shipping: - 11″ 256GB MacBook Air: $798 $301 off original MSRP - 13″ 128GB MacBook Air: $... Read more
5K iMacs on sale for $100 off MSRP, free ship...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. They have the 27″ 3.5GHz 5K iMac on sale for $2199, also $100... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more

Jobs Board

*Apple* TV Live Streaming Frameworks Test En...
**Job Summary** Work and contribute towards the engineering of Apple 's state-of-the-art products involving video, audio, and graphics in Interactive Media Group (IMG) at Read more
Project Manager, WW *Apple* Fulfillment Ope...
…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
Senior Data Scientist, *Apple* Retail - Onl...
**Job Summary** Apple Retail - Online sells Apple products to customers around the world. In addition to selling Apple products with unique services such as iPad Read more
*Apple* Music Producer - Apple (United State...
**Job Summary** Apple Music seeks a Producer to help shepherd some of the most important content and editorial initiatives within the music app, with a particular focus Read more
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.