#17554: Unable to substitute univariate Laurent polynomial into usual polynomial
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Reporter: tscrim | Owner: tscrim
Type: defect | Status: new
Priority: major | Milestone: sage-6.5
Component: commutative algebra | Resolution:
Keywords: Laurent polynomial, | Merged in:
substitution | Reviewers:
Authors: | Work issues:
Report Upstream: N/A | Commit:
Branch: | Stopgaps:
Dependencies: |
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Comment (by nbruin):
The following seems to be the issue
{{{
sage: p() #evaluating with no arguments should probably be a no-op
q^4 + q^2 - 2*q + 3
sage: x() #but it raises an error for laurent polynomials
IndexError: tuple index out of range
}}}
The problem gets triggered by
`sage.rings.polynomial.polynomial_element.Polynomial.__call__`, which for
keyword arguments does:
{{{
P = self.parent()
name = P.variable_name()
if name in kwds:
if len(x) > 0:
raise ValueError("must not specify both a keyword and
positional argument")
a = self(kwds[name])
del kwds[name]
try:
return a(**kwds)
except TypeError:
return a
}}}
The command `return a(**kwds)` fails, because it's effectively
`a(*(),**{})`. Furthermore, it fails with an `IndexError` which doesn't
get caught.
The better solution is probably to amend laurentpolynomial's `__call__` to
do the right thing. Presently, it doesn't support keyword arguments at all
and it expects non-empty arguments. Its implementation is
{{{
def __call__(self, *x):
if isinstance(x[0], tuple):
x = x[0]
return self.__u(x) * (x[0]**self.__n)
}}}
which expects there is at least one argument and doesn't handle keyword
arguments.
--
Ticket URL: <http://trac.sagemath.org/ticket/17554#comment:2>
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