dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Wed, Nov 04, 2009 at 07:05:01PM +0000, Garth N. Wells wrote:
>
>
> Anders Logg wrote:
> > Was this introduced after the changes to DofMap I pushed today?
> >
>
> No. I went back a few changesets and the problem persisted. It works in
> C++, so I hoped that Johan could shed some light on the Python magic.

I think I can. There's nothing magic about it.

The "problem" is that the [] operator in Python is handled by UFL, not
by DOLFIN. This means that U[1] returns a UFL object, namely the first
(second) component of the function U (which will be a UFL Indexed
object).

This can naturally not be saved to file since UFL does not handle
saving to file.

But plotting works just fine, U[1] will be component 1 of U which is
the same as sigma[1], so what you see in the plot is the y-component
of sigma, or actually the projection of this component onto piecewise
linears, since this is what plot() does for objects that are not
GenericFunction. If it is not a GenericFunction, it tries to project
the object which works just fine since U[1] is a UFL
expression.

Johan and I have discussed the possibility of taking over [] from UFL
or at least adding some hook to UFL that let's us do the right thing.
Johan has used a hook in UFL for handling the () operator. This is
what allows us to do things like

  u((0.5, 0.5))

to evaluate a Function in Python.

--
Anders



> Garth
>
>
>
> >> Something is wrong with sub-functions in Python. For the mixed-poisson
> >> demo, doing
> >>
> >>      (sigma, u) = problem.solve().split()
> >>      plot(u)
> >>
> >> the plot looks as expected. Doing
> >>
> >>      U = problem.solve()
> >>      plot(U[1])
> >>
> >> the plot is wrong. Also, it's not possible to do
> >>
> >>      f = File("u.pvd")
> >>      f << U[1]
> >>
> >> I get the message
> >>
> >> TypeError: in method 'File___lshift__', argument 2 of type
> >> 'dolfin::FunctionPlotData const &'
> >>
> >> Garth
> >> _______________________________________________
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> >> DOLFIN-dev@xxxxxxxxxx
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> >>
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