#13814: LazyFamily.__eq__ gives false positives.
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Reporter: cnassau | Owner: sage-combinat
Type: defect | Status: new
Priority: major | Milestone: sage-5.6
Component: combinatorics | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Christian Nassau | Merged in:
Dependencies: | Stopgaps:
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Description changed by cnassau:
Old description:
> `LazyFamily.__eq__` occasionally returns false positives, because it only
> compares function names, not values. This can lead to subtle bugs later,
> like this one:
>
> {{{
> #!python
> fun = lambda i:i
> fam1 = LazyFamily((0,1),fun)
> fun = lambda i:i+6
> fam2 = LazyFamily((0,1),fun)
> fam3 = LazyFamily((2,3),fun)
> d1 = DisjointUnionEnumeratedSets((fam1,fam3))
> d2 = DisjointUnionEnumeratedSets((fam2,fam3))
> for u in (fam1,fam2,fam3,d1,d2):
> print list(u)
> }}}
>
> This gives
>
> {{{
> [0, 1]
> [6, 7]
> [8, 9]
> [0, 1, 8, 9]
> [0, 1, 8, 9]
> }}}
>
> because Sage thinks `fam1 == fam2`. The behaviour can be fixed by setting
>
> {{{
> def noteq(self,other):
> return False
> LazyFamily.__eq__ = noteq
> }}}
>
> I think `__eq__` should *never* give false positives for classes that
> might be hashed. In this case `LazyFamily.__eq__` should
>
> * test function equality if the index sets are finite of the same size
> * return `False` if the both index sets are infinite
New description:
`LazyFamily.__eq__` occasionally returns false positives, because it only
compares function names, not values. This can lead to subtle bugs later,
like this one:
{{{
#!python
fun = lambda i:i
fam1 = LazyFamily((0,1),fun)
fun = lambda i:i+6
fam2 = LazyFamily((0,1),fun)
fam3 = LazyFamily((2,3),fun)
d1 = DisjointUnionEnumeratedSets((fam1,fam3))
d2 = DisjointUnionEnumeratedSets((fam2,fam3))
for u in (fam1,fam2,fam3,d1,d2):
print list(u)
}}}
This gives
{{{
#!python
[0, 1]
[6, 7]
[8, 9]
[0, 1, 8, 9]
[0, 1, 8, 9]
}}}
because Sage thinks `fam1 == fam2`. The behaviour can be fixed by setting
{{{
#!python
def noteq(self,other):
return False
LazyFamily.__eq__ = noteq
}}}
I think `__eq__` should _never_ give false positives for classes that
might be hashed. In this case `LazyFamily.__eq__` should
* test function equality if the index sets are finite of the same size
* return `False` if one of the indexing sets is infinite
--
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13814#comment:1>
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