why doesn't pop/clear call __delitem__ on a dict?

Chris Rebert clp at rebertia.com
Thu Dec 11 09:05:04 CET 2008

On Wed, Dec 10, 2008 at 11:53 PM, Daniel Fetchinson
<fetchinson at googlemail.com> wrote:
> I just found out that if I want to have a custom dict it's not enough
> to overload __getitem__, __setitem__ and __delitem__ because, for
> example, pop and clear don't call __delitem__. I.e. an instance of the
> following will not print 'deleted' upon instance.pop( 'key' ):
> class mydict( dict ):
>    def __setitem__( self, key, value ):
>        print 'set'
>        super( mydict, self ).__setitem__( key, value )
>    def __getitem__( self, key ):
>        print 'get'
>        super( mydict, self ).__getitem__( key )
>    def __delitem__( self, key ):
>        print 'deleted'
>        super( mydict, self ).__delitem__( key )
> Why is this?

For optimization purposes essentially, so that the built-in dict can
be as fast as possible as it is used pervasively in Python.

> what other methods do I have to overload so that
> I get what I expect for all dict operations?

You might consider just subclassing UserDict.DictMixin instead:
It implements the complete dict interface all in terms of provided
__getitem__(), __setitem__(), __delitem__(), and keys() methods.


Follow the path of the Iguana...

> Cheers,
> Daniel

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