# 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 automaticly 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.

Warning

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.