dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Dec 09, 2008 at 08:44:49AM +0100, Johan Hake wrote:
> On Monday 08 December 2008 22:31:58 Martin Sandve Alnæs wrote:
> > 2008/12/8 Johan Hake <hake@xxxxxxxxx>:
> > > 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__.
> >
> > Here, let me fix that for you:
> >
> >   A common way to figure out how to use a
> >   poorly designed and/or documented
> >   class in python, is to look at the args and kwargs in class.__init__.
> >
> > Forcing the user to look at the source code rarely reflects good code.
> 
> I suppose you do not mean that forcing a user to look at the argument list is 
> reflecting bad code design, but rather the other way around. If we have bad 
> code design we may force the user to look at the argument list together with 
> other documentation?
> 
> > But I agree that keyword arguments isn't such a bad thing in python.
> 
> Good!
> 
> > >> 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.
> >
> > Whatever requirements you decide to put on the element signature,
> > it would be nice if they were documented formally (i.e. not as ffc code),
> > and placed in the UFC manual.
> >
> > > 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.
> > >
> > > 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])
> > >
> > > We already have checks for cpp.FunctionSpace and cpp.Function for the
> > > coefficients.
> > >
> > > I do not know how the coefficient kwarg should work though.
> > >
> > >  assemble(...,coefficient ={f_ufl:f_cpp})
> > >
> > > as the form that is sent in to the assemble function is compiled and does
> > > not carry any information about the ufl/ffc coefficient function. Do you
> > > have anything to add here Martin?
> >
> > Looks good. Either a dict or a list should be accepted.
> > UFC does not require a UFL source!
> 
> Exactly.
> 
> > Maybe we can add a helper function:
> >
> > def function_spaces(form, mesh):
> >     spaces = []
> >     ... loop over form arguments and create FunctionSpace objects with
> > reuse if possible
> >     return spaces
> 
> My hands on experience with pure ffc code is nil, so please be more explicit.
> 
> > Maybe not exactly what's needed. But preferably we will have tools
> > that enable easy use of UFC code directly, but which are also
> > used behind the scenes regularly by common code such that
> > they are actually in use and being maintained.
> > Otherwise somebody(tm) will just keep making design decisions
> > that breaks UFC compatibility, which sort of weakens the point of
> > a unified interface...
> 
> I think I see your point. Instead of hiding the ufc stuff, in ffc.jit, which 
> is the case today, we should use it more explicitly in the code, together 
> with hooks to the outside world, for people that want to use PyDOLFIN with 
> pure ufc.
> 
> But isn't this desing desision already broken in C++ DOLFIN?
> 
> Johan

Not necessarily. Before we had an interface to the assembler functions
that allowed sending in ufc objects. This has been removed.

Instead we now provide a simple way to wrap ufc objects into a Form
and then sending it to assemble():

  Form dolfin_form(ufc_form, ...);
  assemble(A, dolfin_form);

(By the way, it looks like we should change the order of arguments in
the Form constructor above so the ufc::form is first.)

This gives the same flexibility as before at the expense of an extra
line of code but simplifies the assembler interface.

-- 
Anders

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