[Python-ideas] Proposal for extending the collections module - bags / multisets, ordered sets / unique lists

[Michael Lenzen]

On 07/18/2009 03:32 PM, Chris Rebert wrote:
> On Fri, Jul 17, 2009 at 6:54 PM, Michael Lenzen<m.lenzen at gmail.com>  wrote:
> <snip>
>> New ABCs (abstract base classes)
>> --------------------------------
> <snip>
>> Mutable - I think there should be one metaclass for all mutable types just
>> like there is currently Iterable, Sized and Container. We could then check
>> instanceof(obj, Mutable). MutableSequence, MutableSet and MutableMapping
>> would inherit Mutable, or we might be able to just do away with them
>> altogether.  There are 3 abstractmethods defined:
>>   * pop(self) - This is the simplest to implement, all mutable classes
>> already do.
>>   * __setitem__(self, key, value) - This is already implemented for list and
>> dict but would have to be defined for set, more later
>>   * __delitem__(self, key) Same as above.
>
> I think this would be better named MutableCollection personally,
> otherwise it's liable to be confused with the more general mutable vs.
> immutable types distinction.
>
> <snip>
>> Extending set to fit the ABCs
>> -----------------------------
>> I mentioned before that sets don't currently fit into my model. Here are my
>> propositions for the three methods that would need to be implemented.
>>   * set.__setitem__(self, elem, value) - Set whether or not elem is in the
>> set based on the boolean evaluation of value. s[elem] = 1 would call
>> s.add(elem) and s[elem] = False would call s.remove(elem), appropriately
>> raising a KeyError if elem not in s. The reason I chose this way is that the
>> only thing you can set about an element of a set is whether or not it is
>> present.
>
> The implicit bool(), which I assume is what you mean by "boolean
> evaluation", here concerns me, as it could mask bugs where something
> tries to treat a set like a mapping. I would recommend throwing
> TypeError if given a non-int and ValueError if given any integer
> besides 0 or 1, so as to more strongly enforce the set axioms.
>
> Cheers,
> Chris

Sounds good to me.  I changed my implementation and the description on 
the wiki.  I also changed bags to enforce the int restriction.

-Lenzen