#18246: remove naive __hash__ from SageObject
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Reporter: | Owner:
vdelecroix | Status: new
Type: | Milestone: sage-6.9
defect | Resolution:
Priority: major | Merged in:
Component: | Reviewers: Volker Braun
coercion | Work issues:
Keywords: | Commit:
Authors: | 4d8ec2a1282b24064f93bc8bea5fe23ec08f1750
Vincent Delecroix | Stopgaps:
Report Upstream: N/A |
Branch: |
public/18246 |
Dependencies: |
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Comment (by nbruin):
Replying to [comment:39 tscrim]:
> I actually don't like these changes:
> {{{#!diff
> -class VectorPartitions(Parent, UniqueRepresentation):
> +class VectorPartitions(UniqueRepresentation, Parent):
> }}}
> Why does `UniqueRepresentation` have to come before `Parent`?
Because `UniqueRepresentation` provides a hash that is fast and
appropriate and `Parent` doesn't:
{{{
sage: Parent.__hash__
<slot wrapper '__hash__' of
'sage.structure.category_object.CategoryObject' objects>
sage: UniqueRepresentation.__hash__
<slot wrapper '__hash__' of 'sage.misc.fast_methods.WithEqualityById'
objects>
}}}
In order to get the MRO right, `UniqueRepresentation` needs to appear
before Parent:
{{{
sage: class A(UniqueRepresentation, Parent): pass
sage: A.__hash__
<slot wrapper '__hash__' of 'sage.misc.fast_methods.WithEqualityById'
objects>
sage: class B(Parent, UniqueRepresentation): pass
sage: B.__hash__
<slot wrapper '__hash__' of
'sage.structure.category_object.CategoryObject' objects>
}}}
Once all default hashes have been deleted and no classes in the MRO of
Parent offer a hash function, the order doesn't matter. Until then,
`UniqueRepresentation` has to come first.
> Also can someone explain to me why are `RiggedConfigurations` now
showing machine dependent iteration order?
The code is hard to follow, but in general, if enumeration somewhere
involves a set or a dictionary, then results depend heavily on the hash
used and the order in which the dictionary was constructed. I'd expect
that a changed hash causes things to be enumerated in a different order
(because a set or dict is storing its entries in a different order
somewhere)
--
Ticket URL: <http://trac.sagemath.org/ticket/18246#comment:40>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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