Looking through the unanswered Python questions on StackOverflow, I found one that seemed interesting.. "Python Get Screen Pixel Value in OS X" - how to access screen pixel values, without the overhead of calling the screencapture command, then loading the resulting image.

After a bit of searching, the best supported way of grabbing a screenshot is provided by the CoreGraphics API, part of Quartz, specifically CGWindowListCreateImage.

Since CoreGraphics is a C-based API, the code map almost directly to Python function calls. It's also simplified a bit, because PyObjC handles most of the memory-management (when the wrapping Python object goes out of scope, the underlying object is freed)