How can I capture iSight frames with Python in Snow Leopard?

I have the following PyObjC script:

from Foundation import NSObject
import QTKit
error = None
capture_session = QTKit.QTCaptureSession.alloc().init()
print 'capture_session', capture_session
device = QTKit.QTCaptureDevice.defaultInputDeviceWithMediaType_(QTKit.QTMediaTypeVideo)
print 'device', device, type(device)
success = device.open_(error)
print 'device open success', success, error
if not success:
    raise Exception(error)
capture_device_input = QTKit.QTCaptureDeviceInput.alloc().initWithDevice_(device)
print 'capture_device_input', capture_device_input, capture_device_input.device()
success = capture_session.addInput_error_(capture_device_input, error)
print 'session add input success', success, error
if not success:
    raise Exception(error)
capture_decompressed_video_output = QTKit.QTCaptureDecompressedVideoOutput.alloc().init()
print 'capture_decompressed_video_output', capture_decompressed_video_output
class Delegate(NSObject):
    def captureOutput_didOutputVideoFrame_withSampleBuffer_fromConnection_(self, captureOutput, videoFrame, sampleBuffer, connection):
        print videoFrame, sampleBuffer, connection
delegate = Delegate.alloc().init()
print 'delegate', delegate
capture_decompressed_video_output.setDelegate_(delegate)
print 'output delegate:', capture_decompressed_video_output.delegate()
success = capture_session.addOutput_error_(capture_decompressed_video_output, error)
print 'capture session add output success', success, error
if not success:
    raise Exception(error)
print 'about to run session', capture_session, 'with inputs', capture_session.inputs(), 'and outputs', capture_session.outputs()
capture_session.startRunning()
print 'capture session is running?', capture_session.isRunning()
import time
time.sleep(10)

The program reports no errors, but iSight's green light is never activated and the delegate's frame capture callback is never called. Here's the output I get:

$ python prueba.py 
capture_session <QTCaptureSession: 0x1006c16f0>
device Built-in iSight <objective-c class QTCaptureDALDevice at 0x7fff70366aa8>
device open success (True, None) None
capture_device_input <QTCaptureDeviceInput: 0x1002ae010> Built-in iSight
session add input success (True, None) None
capture_decompressed_video_output <QTCaptureDecompressedVideoOutput: 0x104239f10>
delegate <Delegate: 0x10423af50>
output delegate: <Delegate: 0x10423af50>
capture session add output success (True, None) None
about to run session <QTCaptureSession: 0x1006c16f0> with inputs (
    "<QTCaptureDeviceInput: 0x1002ae010>"
) and outputs (
    "<QTCaptureDecompressedVideoOutput: 0x104239f10>"
)
capture session is running? True

PS: Please don't answer I should try PySight, I have but it won't work because Xcode can't compile CocoaSequenceGrabber in 64bit.

Your problem here is that you don't have an event loop. If you want to do this as a standalone script, you'll have to figure out how to create one. The PyObjC XCode templates automatically set that up for you with:

from PyObjCTools import AppHelper
AppHelper.runEventLoop()

Trying to insert that at the top of your script, however, shows that something inside AppHelper (probably NSApplicationMain) expects a plist file to extract the main class from. You can get that by creating a setup.py file and using py2app, something like this example from a PyObjc talk:

from distutils.core import setup
import py2app
plist = dict(
    NSPrincipalClass='SillyBalls',
)
setup(
    plugin=['SillyBalls.py'],
    data_files=['English.lproj'],
    options=dict(py2app=dict(
        extension='.saver',
        plist=plist,
    )),
)