dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jan 18, 2010 at 10:23:54AM -0800, Johan Hake wrote:
> On Monday 18 January 2010 08:29:03 dbeacham wrote:
> > New question #97842 on DOLFIN:
> > https://answers.launchpad.net/dolfin/+question/97842
> >
> > Is there a less hacked way to add extra parameter values to be used in an
> >  Expression in python than:
> >
> > class MyExp(Expression):
> >     def __new__(cls, *args, **kwargs):
> >         obj = Expression.__new__(cls, **kwargs)
> >         return obj
> >
> >     def __init__(self, *args):
> >         ....
> >         ....
>
> It has been possible to pass user defined optional arguments to the __init__
> method. But it looks like a regression has kicked in.
>
> I have fixed the regression in dolfin dev, and added a doc string example.
>
> You should now be able to do:
>
>    class MyExp(Expression):
>        def __init__(self, **kwargs):
>            self._mesh = kwargs['mesh']
>            self._domain = kwargs['domain']
>        def eval(self, values, x):
>           info(self._mesh, 1)
>           info(self._domain, 1)
>
>    mesh = UnitCube(1,1,1)
>    domain = MeshFunction("uint", mesh, 0)
>    e = MyExp(mesh=mesh, domain=domain)
>    e(0,0,0)
>
> Do not use *args and do _not_ call the base class __init__. Yes, it is counter
> intuitive but it is a result of having the Expression class do both compiled
> Expressions and python derived Expressions.
>
> I prefer having two classes, CompiledExpression and Expression, so we could
> say goodbye to this kind of troubles ;)

I'd prefer keeping "Expression" for the simplest option which is
visible in most demos:

  f = Expression("sin(x[0])")

and find a suitable name for Expression with eval.

Ideally, it should also be the same in C++.

Candidates are

  UserExpression
  ComplexExpression

But I'm not sure I like any of them.

--
Anders


> Johan
>
>
> > where *args are my extra parameters (ie mesh, mesh functions etc.) and
> >  __new__ might have to be better worked if I had included my own kwargs.
> >  The above seems to work ok, but let me know of any major problems with it.
> >
> > I could always use a MyExp.init(...) function after I've created an
> >  instance to set up class variables, but just curious as to whether I
> >  missed something obvious for a one line initiation.
> >
> > David.
> >
>
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