Overriding iadd for dictionary like objects

[Robert Kern]

On 2009-08-26 20:00 PM, Jan Kaliszewski wrote:
> 27-08-2009 o 00:48:33 Robert Kern <robert.kern at gmail.com> wrote:
>
>> On 2009-08-26 17:16 PM, RunThePun wrote:
>>> 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...
>>
>> I'm sorry, this is just part of the syntax of Python. You cannot
>> override it.
>
> Though
> d['a'] = 3
> is equivalent to:
> d.__setitem__('a', 3)
>
> The
> d['a'] += 3
> *is not* equivalent to:
> d.__setitem__('a', d.__getitem__('a') + 3)
> *but is* equivalent to:
> d.__getitem__('a').__iadd__(3)
>
> Then you can override __getitem__() of MyDict in such a way that it
> returns prepared (wrapped) object with overriden __iadd__() as you
> want to.

You could, but then you will almost certainly run into problems using the 
wrapped object in places that really expect the true object.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco