#15223: Let the `TestSuite` test that the construction of a parent returns the
parent
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Reporter: SimonKing | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-5.12
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by SimonKing):
I did not run the full test suite yet, but I already detected several bugs
in `.construction()`. So, introducing this test really is a good idea.
Until now, I found:
1. Boolean polynomial rings:
{{{
sage: P.<x0, x1, x2, x3> =
BooleanPolynomialRing(4,order='degrevlex(2),degrevlex(2)')
sage: P.construction()
(MPoly[x0,x1,x2,x3], Finite Field of size 2)
sage: F, O = P.construction()
sage: F(O)
Multivariate Polynomial Ring in x0, x1, x2, x3 over Finite Field of size 2
sage: P
Boolean PolynomialRing in x0, x1, x2, x3
}}}
Suggested solution: A boolean polynomial ring is, after all, a quotient
of a polynomial ring. So, it should be constructed as such, but the
`QuotientFunctor` should be provided with an additional attribute making
it notice that a boolean polynomial ring shall be returned.
2. Integer mod rings that are initialised in a non-default category:
{{{
sage: P = IntegerModRing(19, category = Fields())
sage: F,O = P.construction()
sage: F(O)
Ring of integers modulo 19
sage: P
Ring of integers modulo 19
sage: F(O) == P
False
}}}
Why is this? Well, providing a different category changes the class of
P (this is how the category framework works), and the `__cmp__` method of
integer mod rings checks for the class. And the construction functor `F`
is not aware of the category of `P`.
Suggested solutions: Change `__cmp__` such that `F(O)` evaluates equal
to `P`, even though `F(O)` is not in the category of fields, or
alternatively make the construction functor aware of the category, so that
`F(O)` actually ''is'' in the category of fields.
--
Ticket URL: <http://trac.sagemath.org/ticket/15223#comment:1>
Sage <http://www.sagemath.org>
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