dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Dec 02, 2008 at 06:19:18PM +0100, Martin Sandve Alnæs wrote:
> 2008/12/2 Anders Logg <logg@xxxxxxxxx>:
> > On Tue, Dec 02, 2008 at 12:12:14PM +0100, Anders Logg wrote:
> >> I've tried adding a new class Constant in function.py:
> >>
> >> class Constant(ffc.Constant, dolfin.cpp_Function):
> >>
> >>     def __init__(self, domain, value):
> >>         "Create constant-valued function."
> >>         print domain
> >>         print value
> >>         #ffc.Constant.__init__(self, domain)
> >>         #dolfin.cpp_Constant.__init__(self, value)
> >>
> >> But I get the following error message:
> >>
> >>   File
> >>   "/scratch/fenics/dolfin/dolfin-dev/local/lib/python2.5/site-packages/dolfin/function.py", line 411, in <module>
> >>     class Constant(ffc.Constant, dolfin.cpp_Function):
> >> TypeError: Error when calling the metaclass bases
> >>     function() argument 1 must be code, not str
> >>
> >> How is this possible? There should be no metaclasses involved here
> >> (except the built-in Python metaclass type that is always there).
> >
> > I get this error even if I just try to create a class named anything
> > that inherits from ffc.Constant.
> >
> > Does the metaclass construction in function.py have side-effects?
> >
> > I don't remember if we discussed this before, but would it be possible
> > (at least simpler) to instead define a simple Python function that
> > returns a "function" instance:
> >
> > class FooFunction(ffc.Function, ...):
> >    ...
> > class BarFunction(dolfin.Function, ...):
> >    ...
> >
> > def Function(V, *arg):
> >
> >    if foo:
> >        return FooFunction(...)
> >    elif bar:
> >        return BarFunction(...)
> >
> > This seems to be an easier solution. It would still be dynamic.
> 
> The __metaclass__ attribute of Function doesn't have to be a type, it
> can be any callable.
> 
> > The only drawback would be that we can't do
> >
> >  isinstace(v, Function)
> 
> As mentioned before, I see that as an advantage ;)

But if all created classes inherit from FunctionBase, it would be the
same as for UFL/FFC where all elements inherit from FiniteElementBase:

  if isinstance(v, FunctionBase):
      ...

The advantage would be that the code would be easier to understand (at
least I would understand it). Maybe I would also be able to understand
the current implementation if it was reorganized a bit (as suggested
earlier with __new_discrete_function etc).

Where is Hake when you need him? :-)

-- 
Anders

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