simplicial complexes lack hash function

[sagetrac-vpilaud]

Simplicial complexes are lacking a proper hash function. See the example below.

sage: S = SimplicialComplex([[]]); S
Simplicial complex with vertex set () and facets {()}
sage: hash(S)
-3899221226149827755
sage: S.__hash__??
Source:
    def __hash__(self):
        return hash(self.__repr__())

---

Apply:

• attachment: trac_12587-simplicial_complex_hash-ts.patch
• attachment: trac_12587-simplicial_complex_hash_review-cs.patch
• attachment: trac_12587-ref-jhp.patch

Depends on #13226
Depends on #13244
Depends on #13590

CC: @sagetrac-sage-combinat

Component: combinatorics

Author: Travis Scrimshaw

Reviewer: Christian Stump, John Palmieri

Merged: sage-5.6.beta1

Issue created by migration from https://trac.sagemath.org/ticket/12587

[tscrim]

comment:1

Since SimplicialComplex is mutable, the hash function must depend on something immutable. Because of this, I would like for it to be handled like Graph and throw a TypeError. However there are many dependencies on SimplicialComplex being hashable, so my thought behind setting this to wontfix is that repr is the only immutable part of the class.

[jhpalmieri]

comment:2

Why not instead modify the SimplicialComplex class so that it is immutable?

[sagetrac-sluther]

comment:3

Being mutable and hashable is definetly a bug. See [1].

I don't understand what you mean with repr is the only immutable part, because it may change when the object changes.

[1] http://docs.python.org/glossary.html#term-hashable