#20312: parent of argument lost with Functions
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Reporter: rws | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-7.2
Component: symbolics | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Description changed by rws:
Old description:
> Except for some classes, Python objects as function arguments are coerced
> into SR, but with `GinacFunction`s as well as with `BuiltinFunction`s the
> parent gets lost, i.e. the result is not converted back. Ths violates the
> rule that function input and output should be of the same type.
> {{{
> sage: S.<y> = PolynomialRing(RR)
> sage: ex = sin(asin(y)); ex
> y
> sage: type(ex)
> <type 'sage.symbolic.expression.Expression'>
> sage: ex.is_symbol()
> True
> }}}
> This also shows that the notion "SR encapsulates all Sage objects" is
> wrong because everything coerces into SR. SR only encapsulates certain
> "numeric" objects. Pynac can't be blamed as it only sees the coerced
> expressions. The problem is in
> `symbolic/function.pyx:BuiltinFunction.__call__` where the parent of
> function arguments is lost.
>
> See also #18832 and maybe #17790.
New description:
Some Python objects as function arguments lose their parent, i.e. the code
in the specific function can no longer access it because the object was
coerced into SR. This can result in violation of the rule that function
input and output should be of the same type.
{{{
sage: S.<y> = PolynomialRing(RR)
sage: ex = sin(asin(y)); ex
y
sage: type(ex)
<type 'sage.symbolic.expression.Expression'>
sage: ex.is_symbol()
True
}}}
The specific function (here sin) code cannot remedy this because there the
damage is already done. The problem is in
`symbolic/function.pyx:BuiltinFunction.__call__` where the parent of
function arguments is lost.
See also #18832 and maybe #17790.
--
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Ticket URL: <http://trac.sagemath.org/ticket/20312#comment:4>
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