Key-Value Observing and Python

Key-Value Coding and Key-Value Observing are supported by PyObjC. Key-Value Observing is only supported for Cocoa objects though, because it is technically impossible to generate the right events for pure python objects (such as dict or list instances)[1].

  • _[1]: at least not without patching the Python interpreter itself.

PyObjC will automatically call willChangeValueForKey: and didChangeValueForKey: when changing the attribute of an object that is a subclass of NSObject. It is therefore not necessary to call those methods in most use-cases for Key-Value Observing.


The ‘change’ dictionary for observeValueForKeyPath:ofObject:change:context: can be changed after the method call, don’t store a reference to this dictionary but make a copy when you want to use its contents later on.