[Python-Dev] PyDict_SetItem hook

Ronald Oussoren ronaldoussoren at mac.com
Tue Apr 7 17:10:01 CEST 2009

On 3 Apr, 2009, at 0:57, Guido van Rossum wrote:
> The primary use case is some kind of trap on assignment. While this
> cannot cover all cases, most non-local variables are stored in dicts.
> List mutations are not in the same league, as use case.

I have a slightly different use-case than a debugger, although it  
boils down to "some kind of trap on assignment": implementing  Key- 
Value Observing support for Python objects in PyObjC.  "Key-Value  
Observing" is a technique in Cocoa where you can get callbacks when a  
property of an object changes and is something I cannot support for  
plain python objects at the moment due to lack of a callback  
mechanism.   A full implementation would require hooks for mutation of  
lists and sets as well.

The lack of mutation hooks is not a terrible problem for PyObjC, we  
can always  use Cocoa datastructures when using KVO, but it is  
somewhat annoying that Cocoa datastructures leak into code that could  
be pure python just because I want to use KVO.


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