On Monday, February 29, 2016 at 2:56:06 PM UTC-8, William wrote:
>
> Hi,
>
> Somebody doing differential equations sent me this code, which you can
> run in a fresh sage-6.9 session:
>
> var('x,y')
> y = function('y')(x)
> eqn = diff(x^2+y^2==25,x)
> sol0 = solve(eqn, diff(y,x))
> sol1 = sol0[0].subs({x:3})
> sol1.subs({y(3):4})
>
> The *last line* outputs:
>
> D[0](y)(3) == (-3/4)
> DeprecationWarning: Substitution using function-call syntax and
> unnamed arguments is deprecated and will be removed from a future
> release of Sage; you can use named arguments instead, like EXPR(x=...,
> y=...)
> See http://trac.sagemath.org/5930 for details.
> exec compile(block+'\n', '', 'single') in namespace, locals
>
> I have some problems with this:
>
> (1) trac #5930 hasn't been touched in *7 years*, and doesn't mention
> subs anywhere. Maybe referencing 5930 was a typo?
>
The patch mentions subs. I'm not an expert in this, but I think it's the
right ticket.
> (2) The actual substitution above is *NOT* function-call syntax --
> it's passing in a dictionary, which seems like a very reasonable way
> to do substitution in general and should never get deprecated. So why
> is that deprecation warning appearing? Is it actually coming from
> some other code inside the library?
>
Regarding points (2) and (3), the warning comes from the "y(3)" part of the
line, not the "subs" part of the line. I think if you replace the last line
with "sol1.subs({y.subs(x=3):4})", no warning.
> (3) The suggestion to "use named arguments instead, like EXPR(x=...,
> y=...)" seems confusing to me, since it's impossible to specify y(3)
> as a named argument. Also, it assumes that a variable is the same as
> its string, which is a bad assumption to make and just begs for bugs;
> using a dictionary (or ==) is always safer and should be preferred,
> right?
>
> William
>
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