overriding __getitem__ for a subclass of dict

[Christian Heimes]

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).

> 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)

Because dict is the most important and speed critical type in Python it
has some special behaviors. If you are going to overwrite __getitem__ of
a dict subclass then you have to overwrite all methods that call
__getitem__, too. These are get, pop, update and setdefault.

Christian