Dear all,
As I'm sure most people here know, polynomials in Sage are kind of
immutable - they are meant to be immutable and (most) methods are
allowed to assume this to improve their performance, but sometimes
there an interface is (see _unsafe_mutate) to modify a polynomial
nonetheless.
I've recently come across two problems and would like some opinions on
this - I think they are bugs and if there is an agreement of this I'm
happy to write up a patch:
(1) Somewhere in polynomial_element.pyx, quoted from a comment on by
Harald on trac #4000, for some polynomial f in a univariate polynomial
ring with variable x,
{{{
sage: f(x) is f
Expected:
True
Got:
False
}}}
(I'm sorry for the vagueness, but I've first seen this in Harald's
comment, and he's using Sage 4.5.2 RC, which I am building only just
now.) I think while the composition of a polynomial with its variable
*might* be allowed to take advantage of the immutability and return
self, I think it's going over the top to enforce that it actually
does. That is to say, I think the check "f(x) == f" should definitely
return True, however, I don't think "f(x) is f" should be enforced to
return True.
(2) In the same file,
{{{
def __copy__(self):
return self
}}}
I think it's great that many methods can improve their performance by
assuming that polynomials are immutable. However, I think the copy
method should be an exception to this - after all, the caller is
explicitly asking for a "copy".
Thank you for your comments in advance,
Sebastian
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