Le 10/07/2012 21:58, William Stein a écrit :
On Tue, Jul 10, 2012 at 11:01 AM, Martin Raum
<[email protected]> wrote:
Hallo all!
I have found the following bug which mostly occurs when substituting power
series over inexact rings. E.g.
R.<x> = CC[[]]
x.subs(x = x**2)
gives x. Same for the base field Qp(7).
Quick remark. It is far, far better to use a dictionary as input to
subs, if you can stand typing the extra braces. For example, the
above issue works fine with a dictionary:
sage: R.<x> = CC[[]]
sage: x.subs({x:x**2})
0.000000000000000 + 0.000000000000000*x + 1.00000000000000*x^2
The issue is that if you use subs(foo=bar), then Python interprets foo
as the keyword (=string) "foo"; there is *no way* to do subs(x=x^2) in
Python and have x be a Python variable. Here's another example where
subs with a dictionary is much better:
sage: R.<x,x> = CC[[]]; f = R.0 + R.1; f
x + x
sage: f.subs({R.0:5}) == 5 + R.1
True
sage: f.subs({R.1:5}) == 5 + R.0
True
There's no way with strings to ever do the above...
I can't help but notice that help(f.subs) indeed says the argument
should be a dict, but most examples don't do that (in fact, only the
very first one uses a dict!).
Documentation bug?
Snark
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