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

Tinderbox 7.0.0 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
1Password 6.5.5 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
Apple Remote Desktop Client 3.9 - Client...
Apple Remote Desktop Client is the best way to manage the Mac computers on your network. Distribute software, provide real-time online help to end users, create detailed software and hardware reports... Read more
Art Text 3.2.2 - $49.99
Art Text is graphic design software specifically tuned for lettering, typography, text mockups and various artistic text effects. Supplied with a great variety of ready to use styles and materials,... Read more
WhatRoute 2.0.15 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the router... Read more
Sparkle 2.1.1 - $79.99
Sparkle will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
Dash 4.0.1 - 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
TextSoap 8.3.2 - Automate tedious text d...
TextSoap can automatically remove unwanted characters, fix up messed up carriage returns, and do pretty much anything else that we can think of to text. Save time and effort. Be more productive. Stop... Read more
Apple Remote Desktop 3.9 - Remotely cont...
Apple Remote Desktop is the best way to manage the Mac computers on your network. Distribute software, provide real-time online help to end users, create detailed software and hardware reports, and... Read more
Paragraphs 1.1.4 - 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. Features... Read more

Blasty Bubs is a colorful Pinball and Br...
QuickByte Games has another arcade treat in the works -- this time it's a mishmash of brick breaking and Pinball mechanics. It's called Blasty Bubs, and it's a top down brickbreaker that has you slinging balls around a board. [Read more] | Read more »
Corsola and Heracross are the new region...
Generation 2 finally launched in Pokémon GO, unleashing a brand new batch of Pokémon into the wild. Even before the update went live people were speculating on how to catch elusive Pokémon like the legendary "dogs", Unknown, and whether or not... | Read more »
The Warlock of Firetop Mountain (Games)
The Warlock of Firetop Mountain 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: An epic adventure through a mysterious mountain filled with monsters, magic and mayhem! “...it looks downright... | Read more »
Fantasy MMORPG MU Origin’s receives a hu...
Developer Webzen are looking to take their highly popular fantasy battler MU Origin to the next level this month, with its most ambitious overhaul yet. The latest update introduces the long sought after Server Arena, new treasure dungeons, and much... | Read more »
RPG Djinn Caster (Games)
RPG Djinn Caster 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: SPECIAL PRICE 38% OFF(USD 7.99 -> USD 4.99)!!!A Fantasy Action RPG of far foreign lands! Summon the Djinns and rise to... | Read more »
Alto's Odyssey gets its first trail...
There's finally video evidence of Alto's Odyssey, the follow up to the 2015 App Store hit, Alto's Adventure. It looks just as soothing and atmospheric as Alto's last outing, but this time players will be journeying to the desert. Whereas Alto's... | Read more »
Last week on Pocket Gamer
What’s going on in the wider world of portable gaming? Each week we ask that question of our sister website Pocket Gamer. The PG team covers iOS gaming, just like 148Apps, but it also strays into the world of Android games and handheld consoles... | Read more »
Pokémon GO Generation 2 evolution guide
At long last, Niantic Labs finally unleashed the Generation 2 Pokémon into the wild. Pokémon GO trainers are scrambling to grab up this new set of 80 Pokémon. There are some special new tricks required to catch all of these new beasties, though.... | Read more »
The best new games we played this week
It feels as though the New Year got off to a creaking start as far as mobile games go, but that's changed over the past few weeks. The last few days alone have seen the debut of a number of wonderful games, so we thought we'd take the time to... | Read more »
Recruit more scallywags and discover new...
Get ready to show off your sea legs all over again in Oceans & Empires’ new grand update, which aims to make the act of rising to the role of seven seas ruler even more fresh and appealing, thanks to a richness of new content on both iOS and... | Read more »

Price Scanner via MacPrices.net

Apple’s New iPad Ads Don’t Address Pro Users’...
Apple launched a new tranche of iPad Pro TV ads last week addressing actual queries and challenges from the Twitterverse, albeit using actors for the visuals. That’s great. As an iPad fan and heavy... Read more
Free Verbum Catholic Bible Study App For iOS
The Verbum mobile app runs on Logos’ powerful Bible software and is an advanced resource for mobile Catholic study. The Verbum app surrounds the Bible with the Tradition. Verbum comes with 15 free... Read more
27-inch Apple iMacs on sale for up to $200 of...
B&H Photo has 27″ Apple iMacs on sale for up to $200 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099.99 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon has 2015 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) available for $1849.99 including free shipping. Apple charges $1999 for this model, so Amazon’s price is represents a $150 savings. Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 WiFis available for starting at $319 including free shipping. A standard Apple one-year warranty is included: - 16GB iPad Air 2 WiFi: $319 $60 off original... Read more
Apple refurbished iPad Pros available for up...
Apple has Certified Refurbished 9″ and 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 9″... Read more
Apple restocks refurbished 2015 and 2016 13-i...
Apple has Certified Refurbished 2015 and 2016 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB... Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros (MD101LL/A) available for $829, or $270 off original MSRP. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook... Read more
QuickerTek Announces 5TB Apple AC AirPort Tim...
QuickerTek Inc. has announced their new 5TB hard drive upgrade for Apple’s AC AirPort Time Capsule. By customer request, this upgrade also features six external antennas and offers the highest... Read more
Apple Certified Refurbished iMacs available f...
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

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
*Apple* Technician - nfrastructure (United S...
Let’s Work Together Apple Technician This position is based in Portland, ME Life at nfrastructure At nfrastructure, we understand that our success results from our Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.