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?
(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?
(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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