overriding __getitem__ for a subclass of dict

[Steve Howell]

On Nov 15, 4:03 pm, Christian Heimes <li... at cheimes.de> wrote:
> Steve Howell wrote:
> > Does anybody have any links that points to the rationale for ignoring
> > instance definitions of __getitem__ when new-style classes are
> > involved?  I assume it has something to do with performance or
> > protecting us from our own mistakes?
>
> Most magic methods are implemented as descriptors. Descriptors only
> looked up on the type to increase the performance of the interpreter and
> to simply the C API. The same is true for other descriptors like
> properties. The interpreter invokes egg.__getitem__(arg) as
> type(egg).__getitem__(egg, arg).
>

Is the justification along performance lines documented anywhere?

> > So now I am still in search of a way to hook into calls to foo[bar]
> > after foo has been instantiated.  It is all test code, so I am not
> > particularly concerned about safety or future compatibility.  I can do
> > something really gross like monkeypatch Foo class instead of foo
> > instance and keep track of the ids to decide when to override
> > behavior, but there must be a simpler way to do this.
>
> Try this untested code:
>
> class Spam(dict):
>     def __getitem__(self, key):
>         getitem = self.__dict__.get("__getitem__", dict.__getitem__)
>         return getitem(self, key)
> [...]

Not sure how this helps me, unless I am misunderstanding...

It is the futility of writing lowercase_spam.__getitem__ that is
setting me back.  For my use case I do not want to override
__getitem__ for all Spam objects, nor do I even have the option to
modify the Spam class in some cases.