#13814: LazyFamily.__eq__ gives false positives.
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Reporter: cnassau | Owner: sage-combinat
Type: defect | Status: needs_review
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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Comment (by nthiery):
Hi!
Replying to [ticket:13814 cnassau]:
> `LazyFamily.__eq__` occasionally returns false positives, because it
only compares function names, not values.
Yikes. This is indeed definitely a bug!
Thanks for catching it, and sorry for commenting on it after you have
implemented a first proposal of solution.
> I think `__eq__` should _never_ give false positives for classes that
might be hashed.
+1!
> 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
I would much prefer to stick as much as possible to the default
equality rule for composite objects: namely to compare their
components for equality. A Family is composed of an indexing set `I`
and a mapping `f`. Thus, ideally, two families should be equal if
their indexing sets are equal and their mappings are equal. How two
sets (infinite or not) compare for equality is not Family's decision
to take.
Now, we have the problem that comparing mappings is delicate; first
because two functions with the same code are not considered as equal,
and anyway because two functions with different code might implement
the same mapping, but that's undecidable.
So at this point it seems to me that the best approximation would be
as follow:
- For comparing two FiniteFamily's, compare the underlying indexing
set and dictionary for equality (the mapping has already been
computed on all the indexing set, so we might as well use it, even
though this comparison will have a cost).
- For comparing other families: test whether the two indexing sets and
the two mappings compare to equal.
Potential issue: that upon pickling unpickling we get back an equal
object.
Thanks again for investigating this,
Best,
Nicolas
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13814#comment:5>
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