dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Johan Hake wrote:
> On Saturday 17 October 2009 11:48:57 Garth N. Wells wrote:
> > I'd like to create a sub-class of Expression,
> >
> >    class my_expression(Expression):
> >        def __init__(self, V, x):
> >            self.x =x
> >            Expression.__init__(V = V)
> >
> > but I'm having trouble with the constructor - I get the below error. Any
> > tips?
>
> The following should work:
>
> from dolfin import *
>
> class MyExpr(Expression):
>     def __init__(self, x, V=None):
>         self.x = x
>
>     def eval(self, values, x):
>         pass
>
> m = UnitSquare(2,2)
> V = FunctionSpace(m,"CG",1)
>
> MyExpr(0.0, V=V)

Thanks. Works fine. I wouldn't have guessed the solution.

Garth


> This is somewhat inconsistent with how sub classing in Python works. Here you 
> should _not_ call the __init__ method of the base class. 
>
> The short reason is that this is done for you, while instantiating the 
> dynamically created class.
>
> This is something that could be fixed using the suggested new design for 
> CompiledExpression, so one would require an initialization as you suggest 
> above.
>
> Johan
>
> > Garth
> >
> >
> > line 120, in __init__
> >      raise TypeError, "Expression need to be initialized using either a
> > 'cpp.FunctionSpace', using kwarg 'V', or an 'ufl.FiniteElement' using
> > kwarg 'element'."
> > TypeError: Expression need to be initialized using either a
> > 'cpp.FunctionSpace', using kwarg 'V', or an 'ufl.FiniteElement' using
> > kwarg 'element'.
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