[Python-Dev] operator.is*Type

Delaney, Timothy (Tim) tdelaney at avaya.com
Thu Feb 23 00:03:05 CET 2006

Raymond Hettinger wrote:

> Your example simply highlights the consequences of one of Python's
> most basic, original design choices (using getitem for both sequences
> and mappings).  That choice is now so fundamental to the language
> that it cannot possibly change. 

Hmm - just a thought ...

Since we're adding the __index__ magic method, why not have a
__getindexed__ method for sequences.

Then semantics of indexing operations would be something like:

    if hasattr(obj, '__getindexed__'):
        return obj.__getindexed__(val.__index__())
       return obj.__getitem__(val)

Similarly __setindexed__ and __delindexed__.

This would allow distinguishing between sequences and mappings in a
fairly backwards-compatible way. It would also enforce that only indexes
can be used for sequences.

The backwards-incompatibility comes in when you have a type that
implements __getindexed__, and a subclass that implements __getitem__
e.g. if `list` implemented __getindexed__ then any `list` subclass that
overrode __getitem__ would fail. However, I think we could make it 100%
backwards-compatible for the builtin sequence types if they just had
__getindexed__ delegate to __getitem__. Effectively:

    class list (object):

        def __getindexed__(self, index):
            return self.__getitem__(index)

Tim Delaney

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