dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Monday 08 December 2008 22:02:45 Anders Logg wrote:
> On Mon, Dec 08, 2008 at 09:37:44PM +0100, Johan Hake wrote:
> > On Monday 08 December 2008 14:53:09 Anders Logg wrote:
> > > On Mon, Dec 08, 2008 at 02:31:23PM +0100, Johan Hake wrote:
> > > > On Monday 08 December 2008 13:55:26 Anders Logg wrote:
> > > > > On Mon, Dec 08, 2008 at 08:01:32AM +0100, Johan Hake wrote:
> > > > > > On Sunday 07 December 2008 23:33:53 Anders Logg wrote:
> > > > > > > Something that needs to be added to the new Function interface
> > > > > > > is reading functions from file. This has worked before:
> > > > > > >
> > > > > > >   f = Function("function.xml")
> > > > > > >
> > > > > > > Can this be added to the metaclass machinery?
> > > > > >
> > > > > > No, but to the __new__ function ;) so I guess yes!
> > > > > >
> > > > > > The metalclass produces Function classes. The __new__ function
> > > > > > instantiate new Functions from what ever argument. This is such a
> > > > > > case.
> > > > >
> > > > > It looks like some work is needed to get this in place.
> > > > >
> > > > > The "constructor" currently looks as follows:
> > > > >
> > > > >     def __new__(cls, V, **kwargs):
> > > > >
> > > > > so to do
> > > > >
> > > > >     f = Function("function.xml")
> > > > >
> > > > > we need to check if V is a string. Could you add some hooks for
> > > > > this?
> > > >
> > > > I will have a look at it. I do not like V beeing both a FunctionSpace
> > > > and potentially a filename. I consider having a bunch of kwargs, all
> > > > defaulting to None. E.g.
> > > >
> > > >   def __new__(cls, V=None,cpparg=None,defaults=None,filename=None):
> > > >
> > > > Then if you create a Function from file you do:
> > > >
> > > >   f = Function(filename="some_function.xml")
> > > >
> > > > the cpparg can the repreresent what is sent to compile_function.
> > >
> > > Then it would be different from the C++ constructor (which doesn't
> > > handle named default arguments) and the Mesh constructor in both C++
> > > and Python:
> > >
> > >   mesh = Mesh("mesh.xml")
> >
> > The Mesh class has a much clearer C++ interface. It is either
> > instantiated with a filename-string or from another mesh. The Function
> > class is a versatile class already in the C++ interface, which defines
> > different constructors, but evenmore so in the python interface.
> >
> > Then I do not think that adding a kwarg for filename is that bad. The
> > design goal of having as similare interface as possible is good, but then
> > there are different programming cultures too, to take into acount. I am
> > not religious about it, but as far as we can I think we should define
> > kwargs to reflect different instantiation protocol.
> >
> > kwargs gives information about how to instantiate a Function. A common
> > way to figure out how to use a class in python, is to look at the args
> > and kwargs in class.__init__.
>
> ok, sounds reasonable. But didn't you discuss recently to add some
> magic to differentiate between cppcode and cppexpr by some regexping
> to enable
>
>   f = Function(V, "sin(x[0])")
>

Thats true. It is not allways easy to be consistant :) 

But there is a difference between having one argument for either a 
FunctionSpace or a filename, and one argument for anything that can be 
related to the jit compiled functionality.

The only trouble I have now is that all kwargs defaults to None, and one could 
then think that:

  f = Function()

would be possible, which it is not. A nice error message should do the trick?

> Then it would be equally simple to grep for ".xml" or similar.

A lot is doable, but not everything should be done. Sounds like a nice quote 
btw ;)

[snip]

> > > > > We also need to change the signature strings so that one may
> > > > > create form compiler elements (and dofmaps) from the strings.
> > > > >
> > > > > The simplest option could be to let the signature be something like
> > > > >
> > > > >    FiniteElement("Lagrange", "triangle", 1)
> > > >
> > > > So you mean substituting the present
> > > >
> > > >   "Lagrange finite element of degree 1 on a triangle"
> > > >
> > > > with the above? Won't this signature be lost in the parsing of the
> > > > XMLFile, or do you meen that we should parse the function.xml file
> > > > after we have sent it to cpp.Function() and then extract the element
> > > > signature?
> > >
> > > Yes, first create the cpp.Function, then extract
> > > v.function_space.element().signature() and use that to create the form
> > > compiler element. Will that work?
> >
> > Yes it will be doable. I didn't know that the FiniteElements in the
> > elementlibrary all defined its signature. To accomplish this we need to
> > add some construction options for the dolfin.FunctionSpace though, as it
> > is too restrictive now.
>
> ok.

Some more kwargs? :)

> > Martin has also asked for a way to instantiate a dolfin.FunctionSpace
> > either with an ufc form or from an ufc_finite_element together with an
> > ufc_dofmap. We should not forget the pure ufc interface. I think it is
> > good that Martin keep up that pressure! To reverse engeneer a
> > dolfin.FunctionSpace from these entities will probably not be doable.
>
> Why from a form? That looks like what we had before, which we agreed
> was crazy (the reason for getting started on all this work with
> implementing a FunctionSpace class).

Martin?

> > But for that usercase it should be sufficient to construct a
> > cpp.FunctionSpace which can be sent to assemble, e.g:
> >
> >   V = cpp.FunctionSpace(mesh,ufc_element,ufc_dofmap)
> >   f = Function(V,...) # We need to add support for sending in just
> >                       # a cpp.FunctionSpace to a Function, and then
> > return # a cpp.Function
> >
> >   A = assemble(compiled_ufc_form,function_spaces=V,coefficients=[f])
>
> Looks good.


Johan