Overriding iadd for dictionary like objects

[RunThePun]

I'd like to build a database wrapper using DictMixin and allow items
to be appended by my own code. The problem is += is always understood
as setitem and getitem plainly.

d = MyDict()
d['a'] = 1

# this is the problem code that's I'd like to override. It's always
setitem('a', getitem('a') + 3)
d['a'] += 3
# i wanted to do something like my own 'appenditem' function which for
example could be useful if getitem is an expensive operation which can
be avoided.

I hope that was clear enough of a request, it's really late at night
here...

thanks,

RunPun