[Python-Dev] Proposal: add odict to collections

[Michael Foord]

Armin Ronacher wrote:
> Hi,
>
> I noticed lately that quite a few projects are implementing their own
> subclasses of `dict` that retain the order of the key/value pairs.
> However half of the implementations I came across are not implementing
> the whole dict interface which leads to weird bugs, also the performance
> of a Python implementation is not that great.
>
>   

I'm +1 - but this proposal has been made many times before and people 
always argue about what features are needed or desirable. :-(

Michael Foord

> To fight that problem I want to proposed a new class in "collections"
> called odict which is a dict that keeps the items sorted, similar to
> a PHP array.
>
> The interface would be fully compatible with dict and implemented as
> dict subclass.  Updates to existing keys does not change the order of
> a key but new keys are inserted at the end.
>
> Additionally it would support slicing where a list of key, value tuples
> is returned and sort/reverse/index methods that work like their list
> equivalents.  Index based lookup could work via odict.byindex().
>
> An implementation of that exists as part of the ordereddict implementation
> which however goes beyond that and is pretty much a fork of the python
> dict[1].
>
> Some reasons why ordered dicts are a useful feature:
>
>   - in XML/HTML processing it's often desired to keep the attributes of
>     an tag ordered during processing.  So that input ordering is the
>     same as the output ordering.
>
>   - Form data transmitted via HTTP is usually ordered by the position
>     of the input/textarea/select field in the HTML document.  That
>     information is currently lost in most Python web applications /
>     frameworks.
>
>   - Eaiser transition of code from Ruby/PHP which have sorted
>     associative arrays / hashmaps.
>
>   - Having an ordered dict in the standard library would allow other
>     libraries support them.  For example a PHP serializer could return
>     odicts rather then dicts which drops the ordering information.
>     XML libraries such as etree could add support for it when creating
>     elements or return attribute dicts.
>
> Regards,
> Armin
>
>
> [1]: http://www.xs4all.nl/~anthon/Python/ordereddict/
>
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