[Python-Dev] Unordered tuples/lists

[Guido van Rossum]

This is typically called a "bag". Maybe searching for that will help
you find a recipe?

On Tue, May 18, 2010 at 3:13 PM, Gustavo Narea <me at gustavonarea.net> wrote:
> Hello, everybody.
>
> I've been searching for a data structure like a tuple/list *but* unordered --
> like a set, but duplicated elements shouldn't be removed. I have not even
> found a recipe, so I'd like to write an implementation and contribute it to
> the "collections" module in the standard library.
>
> This is the situation I have: I have a data structure that represents an
> arithmetic/boolean operation. Operations can be commutative, which means that
> the order of their operands don't change the result of the operation. This is,
> the following operations are equivalent:
>    operation(a, b, c) == operation(c, b, a) == operation(b, a, c)
>    operation(a, b, a) == operation(a, a, b) == operation(b, a, a)
>    operation(a, a) == operation(a, a)
>
> So, I need a type to store the arguments/operands so that if two of these
> collections have the same elements with the same multiplicity, they are
> equivalent, regardless of the order.
>
> A multiset is not exactly what I need: I still need to use the elements in the
> order they were given. For example, the logical conjuction (aka "and"
> operator) has a left and right operands; I need to evaluate the first/left one
> and, if it returned True, then call the second/right one. They must not be
> evaluated in a random order.
>
> To sum up, it would behave like a tuple or a list, except when it's compared
> with another object: They would be equivalent if they're both unordered
> tuples/lists, and have the same elements. There can be mutable and immutable
> editions (UnorderedList and UnorderedTuple, respectively).
>
> I will write a PEP to elaborate on this if you think it'd be nice to have. Or,
> should I have written the PEP first?
>
> Cheers,
> --
> Gustavo Narea <xri://=Gustavo>.
> | Tech blog: =Gustavo/(+blog)/tech  ~  About me: =Gustavo/about |
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-- 
--Guido van Rossum (python.org/~guido)