#15919: Notation S[i] for i-th element of enumerated set S conflicts with
notation
R[x] for polynomial ring
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Reporter: darij | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.2
Component: algebra | Resolution:
Keywords: notation, algebra, polynomial | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nbruin):
I think the bug is in unrank, not in the fact that rings use `__getitem__`
in a funny way. The problem is that a Cartesian product can easily be
constructed from iterables, and then should probably be iterable itself,
but iterables are not necessarily indexable:
{{{
sage: V={1,2,3}
sage: CartesianProduct(V,V).unrank(2)
TypeError: 'set' object does not support indexing
sage: [i for i in CartesianProduct(V,V)]
[[1, 1], [1, 2], [1, 3], [2, 1], [2, 2], [2, 3], [3, 1], [3, 2], [3, 3]]
}}}
The function "unrank" should only be applied to indexable cartesian
products, i.e., cartesian products of indexables. That's not the same as
iterables. In particular,
So the problem is really in `tester.some_elements`. Indeed, the offending
code:
{{{
try:
n = _len(S)
if n > self._max_runs:
from random import sample
S = sample(S, self._max_runs)
except (TypeError, AttributeError):
}}}
tries `sample` on `S`, which apparently leads to calls of the type `S[i]`.
As you can see, the code is prepared to fail, but doesn't expect that to
happen with a `ValueError`. I think this just shows that it's a bit
callous to just try and index a structure and hope for the best.
--
Ticket URL: <http://trac.sagemath.org/ticket/15919#comment:2>
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