#17427: x==y while hash(x)!=hash(y) with SchemeMorphism_point_projective_field
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Reporter: ncohen | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.5
Component: PLEASE CHANGE | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nbruin):
Replying to [comment:3 bhutz]:
> Although, I'm not sure why this is bad.
It's against contract. It will make behaviour unpredictable if you use
these objects as keys in a dictionary. Python will initially select a
place in the dictionary based on some bits in the hash value, but if that
spot is taken, python will test if the keys agree by testing for equality.
So whether
{{{
{P(1,1):1, P(n,n):2}
}}}
leads to a dictionary with one or two elements will depend on the value of
n. It's really hard to force hash collisions in python so the behaviour is
very rare, which makes it even more dangerous.
Without`a==b` implies `hash(a)==hash(b)` a hash is useless (it's the only
property a hash MUST have)
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Ticket URL: <http://trac.sagemath.org/ticket/17427#comment:4>
Sage <http://www.sagemath.org>
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