#19016: Better hash for Element
-------------------------------------+-------------------------------------
Reporter: ncohen | Owner:
Type: defect | Status: needs_work
Priority: critical | Milestone: sage-6.9
Component: misc | Resolution:
Keywords: | Merged in:
Authors: Nils Bruin, | Reviewers:
Vincent Delecroix | Work issues:
Report Upstream: N/A | Commit:
Branch: public/19016 | a0f830bf8dc0cbdeb75e45ee7bfee7b4fad33768
Dependencies: | Stopgaps:
-------------------------------------+-------------------------------------
Changes (by vdelecroix):
* dependencies: #18246 =>
Old description:
> As reported on sage-devel [1], `Element` implements its hash based on its
> string representation. This causes a lot of troubles
>
> - it breaks the {{{equality => same hash}}} assumption for finitely
> presented groups
> {{{
> sage: G = groups.presentation.Cyclic(4)
> sage: G.cayley_graph().vertices()
> [1, a, a^2, a^-2, a^3, a^-1]
> }}}
> and symbolic expressions
> {{{
> sage: f=sin(x)^2
> sage: g=1-cos(x)^2
> sage: bool(f == g)
> True
> sage: hash(f) == hash(g)
> False
> }}}
> and possibly many others
>
> - it is highly incompatible with the `rename` feature (see also #8119)
> {{{
> sage: from sage.structure.element import Element
> sage: class E(Element):
> ....: def __init__(self):
> ....: Element.__init__(self, Parent())
> sage: e = E()
> sage: hash(e)
> -4965357552728411610
> sage: e.rename('hey')
> sage: hash(e)
> -6429308858210906323
> }}}
> and similarly, hashing should not be available on any mutable object.
>
> - it might be very bad for performance: see #18215 and #18239 for
> examples
>
> There are several possibilities that are currently being discussed:
> - make it return `0` by default (original proposition of the ticket)
> - make it raise a `TypeError` by default (as it the case for
> `SageObject`, see #18246)
> - let it as it is but raise a Warning
>
> [1] https://groups.google.com/d/topic/sage-devel/6rXKkF87Gtc/discussion
New description:
As reported on sage-devel [1], `Element` implements its hash based on its
string representation. This causes a lot of troubles
- it breaks the {{{equality => same hash}}} assumption for finitely
presented groups
{{{
sage: G = groups.presentation.Cyclic(4)
sage: G.cayley_graph().vertices()
[1, a, a^2, a^-2, a^3, a^-1]
}}}
and symbolic expressions
{{{
sage: f=sin(x)^2
sage: g=1-cos(x)^2
sage: bool(f == g)
True
sage: hash(f) == hash(g)
False
}}}
and possibly many others
- it is highly incompatible with the `rename` feature (see also #8119)
{{{
sage: from sage.structure.element import Element
sage: class E(Element):
....: def __init__(self):
....: Element.__init__(self, Parent())
sage: e = E()
sage: hash(e)
-4965357552728411610
sage: e.rename('hey')
sage: hash(e)
-6429308858210906323
}}}
and similarly, hashing should not be available on any mutable object.
- it might be very bad for performance: see #18215 and #18239 for examples
There are several possibilities that are currently being discussed:
- make it return `0` by default (original proposition of the ticket)
- make it raise a `TypeError` by default (as it the case for `SageObject`,
see #18246)
- let it as it is but raise a Warning
[1] https://groups.google.com/d/topic/sage-devel/6rXKkF87Gtc/discussion
See also: #19302, #19321, #19331
--
--
Ticket URL: <http://trac.sagemath.org/ticket/19016#comment:78>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
--
You received this message because you are subscribed to the Google Groups
"sage-trac" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at http://groups.google.com/group/sage-trac.
For more options, visit https://groups.google.com/d/optout.
