Using plists from Python

Python is my current scripting-language-of-choice for a number of reasons, but one of them is that I can handle plists easily, including complex ones, without having to worry about the format that they are in (xml, binary, or even old-style NeXT). I should put the caveat up-front here that this will only work in 10.5 and later, but at this point I don't touch 10.4 machines, and don't anticipate ever working with 10.3 again. So if you can work with that then this method might be for you.

I use the Cocoa bridge to get access to MacOS X's native Foundation layer and the native plist processing available to Obj-C programmers. I know a few other scripter/programmers who use similar techniques in their work, but so far everyone else has been using NSDictionary's dictionaryWithContentsOfFile method. This is great and works well for most plists that you will use, but there are two things you lose by using it:

1. It will read in all of the native plist formats, but the you don't know what format you started with. I like writing things back down in the format I found them in. It probably does not ever matter, but what can I say? In my job I am a little anal about things like this.
2. Not all plists have a dict as their root, some have NSArrays. You probably know going in what the format of the plist you are working with should be, so this is not such a big deal, but I like to be able to be a little more specific about what went wrong why my programs bail.

The solution for these two issues is to use the NSPropertyListSerialization class to read from, and write out your plists. This is easy to do, and the best explanation of it is to give an example, first a minimal one:

#!/usr/bin/python

pathToPlist = [insert path here]
plistNSData, errorMessage = Foundation.NSData.dataWithContentsOfFile_options_error_(pathToPlist, Foundation.NSUncachedRead, None)
plistContents, plistFormat, errorMessage = Foundation.NSPropertyListSerialization.propertyListFromData_mutabilityOption_format_errorDescription_(plistNSData, Foundation.NSPropertyListMutableContainers, None, None)

# plistContents is now a tree with the data

plistNSData, errorMessage = Foundation.NSPropertyListSerialization.dataFromPropertyList_format_errorDescription_(plistContents, plistFormat, None)
suceeeded, errorMessage = plistNSData.writeToFile_options_error_(pathToPlist, Foundation.NSUncachedRead, None)

Important note: Blogger is probably cutting off the ends of lines on the display, and wrapping others. But a copy-and-paste should get you what you need. You also have to fill in the path to your plist of choice there, and this does not do anything other than read the plist, and write it back down unchanged. But if you are looking for a quick cut-and-paste that is probably what you want.

For a more complicated example lets make sure that Acrobat has not been set as the default handler for PDFs. This pulls out most of the stops and checks for all types of problems, so should be a much better example to follow for production code:

#!/usr/bin/python

'''This script sets the default file opener for PDFs to Preview'''

import os, sys, Foundation

# get the path to this user's LaunchServices preference file
pathToLaunchServicesPlist = os.path.expanduser("~/Library/Preferences/com.apple.LaunchServices.plist")
if not os.path.isfile(pathToLaunchServicesPlist):
 raise Exception("The LaunchServices preferences file seems missing: %s" % pathToLaunchServicesPlist)

# read out the data in the file
plistNSData, errorMessage = Foundation.NSData.dataWithContentsOfFile_options_error_(pathToLaunchServicesPlist, Foundation.NSUncachedRead, None)
if errorMessage is not None or plistNSData is None:
 raise Exception("Unable to read in the data from the plist file: %s\nRecived error message: %s" % (pathToFinderPlist, errorMessage))

# convert the data into a useable form
launchServicesPreferences, plistFormat, errorMessage = Foundation.NSPropertyListSerialization.propertyListFromData_mutabilityOption_format_errorDescription_(plistNSData, Foundation.NSPropertyListMutableContainers, None, None)
if errorMessage is not None or pathToLaunchServicesPlist is None:
 raise Exception("Unable to read the data as a plist: %s\nRecived error message: %s" % (pathToLaunchServicesPlist, errorMessage))

# launchServicesPreferences is now a tree of objects that we can modify with normal python methods
#   but we have to check to make sure it looks like we expect

# check to make sure that the root is a dict like we expect it to be
# Note that the root is actually a NSDictionary object,
# but this is bridged to work everywhere at python dict object would.
# But it is not actually a dict object
if not hasattr(launchServicesPreferences, "has_key"):
 raise Exception("The plist does not have a dictionary as its root as expected: %s" % pathToLaunchServicesPlist)

# confirm the LSHandlers item at the first level, and that it reacts like a python list (really a bridged NSArray)
if not "LSHandlers" in launchServicesPreferences or not hasattr(launchServicesPreferences["LSHandlers"], "append"):
 raise Exception("The plist is missing the LSHandlers section, or it was not an array: %s" % pathToLaunchServicesPlist)

# iterate over the array to find any that set the handler for pdfs
for handlerSetting in launchServicesPreferences["LSHandlers"]:
 if hasattr(handlerSetting, "has_key") and "LSHandlerContentType" in handlerSetting and handlerSetting["LSHandlerContentType"] == "com.adobe.pdf":
  handlerSetting["LSHandlerRoleAll"] = "com.apple.preview"

# the setting (if it was set) should now be changed in our in-memory version, we only need to save this back to disk

# convert the tree back to a NSData using the same format we read it in with
plistNSData, errorMessage = Foundation.NSPropertyListSerialization.dataFromPropertyList_format_errorDescription_(launchServicesPreferences, plistFormat, None)
if errorMessage is not None or plistNSData is None:
 raise Exception("Unable to sealize preferences data. Got error message: %s\nTrying to seraliza data:\n%s" % (errorMessage, launchServicesPreferences))

# write the data back down to disk
suceeeded, errorMessage = plistNSData.writeToFile_options_error_(pathToLaunchServicesPlist, Foundation.NSUncachedRead, None)
if errorMessage is not None and suceeeded == True:
 raise Exception("Unable to write preferences back to disk to: %s\nRecieved error message: %s" % (pathToLaunchServicesPlist, errorMessage))

sys.exit(0)