#16280: Fix FiniteEnumeratedSet's call to accept Python objects as input
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Reporter: nthiery | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.2
Component: combinatorics | Resolution:
Keywords: | Merged in:
Authors: Nicolas M. Thiéry | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Changes (by {'newvalue': u'Nicolas M. Thi\xe9ry', 'oldvalue': ''}):
* cc: sage-combinat, ncohen, vdelecroix (added)
* component: PLEASE CHANGE => combinatorics
* type: PLEASE CHANGE => defect
* author: => Nicolas M. Thiéry
Old description:
New description:
*Problem*
Before this ticket, a FiniteEnumeratedSet could not be called on it's
own elements whenever they were not Element's:
{{{
sage: F = FiniteEnumeratedSet(["a", 1])
sage: F(1)
1
sage: F("a")
...
TypeError: Cannot convert str to sage.structure.element.Element
}}}
This prevented the use of `F(x)` as generic idiom to convert `x` into
`F` while checking that it's in `F`.
And indeed:
{{{
sage: TestSuite(F).run()
Failure in _test_an_element:
...
TypeError: Cannot convert str to sage.structure.element.Element
------------------------------------------------------------
The following tests failed: _test_an_element
}}}
*Analysis*
`Parent.__call__` enforces that `_element_constructor_` return an
`Element` (more precisely, it calls `_element_constructor_` through a
`DefaultConvertMap`, and any `Map` requires its results to be
instances of `Element`).
*Proposed solution*
Since `FiniteEnumeratedSets` is often a facade over plain Python
objects, this ticket works around this limitation by a custom
`FiniteEnumeratedSets.__call__` that calls directly
`_element_constructor_` whenever ``el`` is not an `Element`. Otherwise
`Parent.__call__` is called as usual.
*Limitation*
This workaround prevents conversions or coercions from facade parents
over plain Python objects. But it's already much better than before!
--
--
Ticket URL: <http://trac.sagemath.org/ticket/16280#comment:1>
Sage <http://www.sagemath.org>
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