dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

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)

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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