Combinations should be hashable

[maxale]

Steps To Reproduce

We have

sage: hash( Permutations(5).random_element() )
-6659793982185131405
sage: hash( Partitions(5).random_element() )
3863035679738500442
sage: hash( Tuples(range(5),2).random_element() )
7204814214171964562

and so on, except for

sage: hash( Combinations(5,2).random_element() )
...
TypeError: unhashable type: 'list'

Expected Behavior

Combinations should be hashable. Perhaps, they should simply produce elements of type tuple not list.

Actual Behavior

They produce lists that are unhashable.

Additional Information

No response

Environment

• OS: Ubuntu 24.04.1 LTS
• Sage Version: 10.6.beta1

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[DaveWitteMorris]

I don't think there is any statement in the documentation that Combinations are hashable, so this seems to be an enhancement request, not a bug.

[devansh-srv]

Hey @maxale , please assign this issue to me

[maxale]

@devansh-srv: no need to be assigned, just contribute a fix if you have one.

[devansh-srv]

@maxale
The function defined in SAGE_ROOT/src/categories/finite_enumerated_sets.py

        def _random_element_from_unrank(self):
            """
            A random element in ``self``.

            ``self.random_element()`` returns a random element in
            ``self`` with uniform probability.

            This is the default implementation from the category
            ``EnumeratedSet()`` which uses the method ``unrank``.

            EXAMPLES::

                sage: C = FiniteEnumeratedSets().example()
                sage: n = C.random_element()
                sage: n in C
                True

                sage: n = C._random_element_from_unrank()
                sage: n in C
                True

            TODO: implement _test_random which checks uniformity
            """
            from sage.misc.prandom import randint
            c = self.cardinality()
            r = randint(0, c-1)
            return self.unrank(r)

if this function returns
tuple(self.unrank(r)) instead of self.unrank(r) then the issue39455 would be pretty much solved
but this may affect some other part of the codebase as I am making change in the finite_enumerated_set module
rather than this I think some change should be made directly to SAGE_ROOT/src//sage/combinat/combination.py to make it more robust

[maxale]

@devansh-srv: Please note that random elements were taken just as an example, but hashability should be a property of any elements from Combinations class. Fixing a function that produces a random element will not solve the issue in general.

[devansh-srv]

How about introducing some kind of wrapper class to store everything as list but create a tuple copy while hashing

[maxale]

@devansh-srv: I do not know much about Sage internals, but I suspect that a wrapper class may bring an unwelcome overhead.