For the bug, in the following example proposed by Vegrad LIma, the 1st and
last line look contradictory in a dynamical language.
*sage: a = var('a') sage: a.is_real() False*
sage: assume(a,"real")
sage: a.is_real()
False
sage: a = var('a',domain='real')
sage: a.is_real()
True
sage: a = 2+2*I
sage: a.is_real()
False
*sage: a = var('a') sage: a.is_real() True*
*Christophe BAL*
*Enseignant de mathématiques en Lycée **et développeur Python amateur*
*---*
*French math teacher in a "Lycée" **and **Python **amateur developer*
2015-01-29 23:55 GMT+01:00 William Stein <wst...@gmail.com>:
> On Thu, Jan 29, 2015 at 4:58 AM, Christophe Bal <projet...@gmail.com>
> wrote:
> > Thanks for the argument domain.
> >
> > For the end of the code, this sounds like a bug.
>
> No -- I thought somebody would answer, but nobody did. The following
> is definitely *not* a bug:
>
> >> sage: a = 2+2*I
> >> sage: a.is_real()
> >> False
>
> What's happening here is that you assign a 2+2*I, which is complex.
> At this point the variable a in Python is a reference to the symbolic
> expression 2+2*I. It has absolutely nothing whatsoever to do with
> what a was equal to when you typed:
>
> >> sage: a = var('a',domain='real')
> >> sage: a.is_real()
> >> True
>
> This is because Python is a *dynamically* typed language. Thinking
> that doing something like
>
> a = var('a',domain='real')
>
> would have any impact on what
>
> a = BLAH
>
> means would only make sense in a statically typed programming
> language, such as C++ or Java.
>
> -- William
>
>
>
>
>
> >
> >
> > Christophe BAL
> > Enseignant de mathématiques en Lycée et développeur Python amateur
> > ---
> > French math teacher in a "Lycée" and Python amateur developer
> >
> > 2015-01-29 13:04 GMT+01:00 Vegard Lima <vegard.l...@gmail.com>:
> >>
> >> Hi,
> >>
> >> On Thu, Jan 29, 2015 at 12:33 PM, Christophe Bal <projet...@gmail.com>
> >> wrote:
> >> > I've just tried
> >> >
> >> > a = var("a")
> >> > assume(a, "real")
> >> >
> >> > print a.is_real()
> >> >
> >> > This prints False. Why ?
> >>
> >> I can't answer why, but an option is to use
> >> var('a',domain='real')
> >> but that seems to behave a bit strangely:
> >>
> >> sage: a = var('a')
> >> sage: a.is_real()
> >> False
> >>
> >> sage: assume(a,"real")
> >> sage: a.is_real()
> >> False
> >>
> >> sage: a = var('a',domain='real')
> >> sage: a.is_real()
> >> True
> >>
> >> sage: a = 2+2*I
> >> sage: a.is_real()
> >> False
> >>
> >> sage: a = var('a')
> >> sage: a.is_real()
> >> True
> >>
> >>
> >> Cheers,
> >> --
> >> Vegard Lima
> >>
> >> --
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> >
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>
>
> --
> William Stein
> Professor of Mathematics
> University of Washington
> http://wstein.org
>
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