Using PyObjC and NSWorkspace to set the desktop picture. I am such a hypocrite!

gistfile1.py

#!/usr/bin/python

'''Uses Cocoa classes via PyObjC to set a random desktop picture on all screens.
Tested on Mountain Lion and Mavericks.

See:
https://developer.apple.com/library/mac/documentation/cocoa/reference/applicationkit/classes/NSWorkspace_Class/Reference/Reference.html

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html

https://developer.apple.com/library/mac/documentation/cocoa/reference/applicationkit/classes/NSScreen_Class/Reference/Reference.html
'''

import glob
import random

from AppKit import NSWorkspace, NSScreen
from Foundation import NSURL

pictures_glob = glob.glob("/Library/Desktop Pictures/*.jpg")
picture_path = random.choice(pictures_glob)

# generate a fileURL for the desktop picture
file_url = NSURL.fileURLWithPath_(picture_path)

# make image options dictionary
# we just make an empty one because the defaults are fine
options = {}

# get shared workspace
ws = NSWorkspace.sharedWorkspace()

# iterate over all screens
for screen in NSScreen.screens():
    # tell the workspace to set the desktop picture
    (result, error) = ws.setDesktopImageURL_forScreen_options_error_(
                file_url, screen, options, None)

And that is why PyObjC is great.

Not sure if this is supposed to work on 10.6.8 or not, but...

Traceback (most recent call last):
File "./tmp/SetDesktopPictures.py", line 8, in
from AppKit import NSWorkspaceDesktopImageScalingKey
ImportError: cannot import name NSWorkspaceDesktopImageScalingKey


Not tested on Snow Leopard. Feel free to modify to work on SL!

The API is all there in 10.6, but it looks like there are some issues with how the version of PyObjC bundled with 10.6 expose it.

#!/usr/bin/python

# https://developer.apple.com/library/mac/documentation/cocoa/reference/applicationkit/classes/NSWorkspace_Class/Reference/Reference.html
# https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html
# https://developer.apple.com/library/mac/documentation/cocoa/reference/applicationkit/classes/NSScreen_Class/Reference/Reference.html

from AppKit import NSWorkspace, NSScreen
try:
    from AppKit import NSWorkspaceDesktopImageScalingKey
except ImportError:
    NSWorkspaceDesktopImageScalingKey = u'NSWorkspaceDesktopImageScalingKey'
from AppKit import NSImageScaleProportionallyUpOrDown
try:
    from AppKit import NSWorkspaceDesktopImageAllowClippingKey
except ImportError:
    NSWorkspaceDesktopImageAllowClippingKey = u'NSWorkspaceDesktopImageAllowClippingKey'
from Foundation import NSURL

picture_path = "/Library/Desktop Pictures/Tiles Blue.jpg"

# generate a fileURL
file_url = NSURL.fileURLWithPath_(picture_path)

# make image options dictionary
options = {
    NSWorkspaceDesktopImageScalingKey: NSImageScaleProportionallyUpOrDown,
    NSWorkspaceDesktopImageAllowClippingKey: True
}

# get shared workspace
ws = NSWorkspace.sharedWorkspace()

# iterate over all screens
for screen in NSScreen.screens():
    # tell the workspace to set the desktop picture
    # on 10.6 this returns 0 (failure) or 1 (success)
    # on 10.8 this returns a tuple (result, error); success is (True, None)
    result = ws.setDesktopImageURL_forScreen_options_error_(
        file_url, screen, options, None)

Tested on Lion. Works great. Thanks, Greg!