dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Friday 27 November 2009 23:59:52 Johan Hake wrote:
> Hello!
> 
> I have not pushed anything, but Expressions should be up to shape now, and
>  I am working on simplifying the logic in other places of expression.py
> 
> I do not like the implementation of the dim method. It is used to read a
>  user defined value dimension and then discarded. I suggest that we add a
>  value_shape attribute to the dolfin.Expression class. If not set by the
>  user it is by default (), i.e., a scalar Expression.
> 
>   class MyExpression(Expression):
>       value_shape = (2,)
>       def eval(self, values, x):
>          values[0] = 1
>          values[1] = 1
> 
> The dim name is also only making sense for Vector Expressions, where I
>  think a value_shape attribute makes sense for all occasions. A value_shape
>  for scalar Expression will still be the default.
> 
> After creation and instantiation we then have an object that have both a
>  shape method (from ufl.Function) and a value_shape attribute from the
>  subclassing definition.
> 
> What you think.

Hmm, more trouble I think. How should we be able to specify the degree, and or 
element when we have derived python Expression? Should we add more attributes?

One way is to adapt the CompiledExpression solution, and to make 
dolfin.Expression a pure and nice subclass of ufl.Function and cpp.Expression. 
a user defined python Expression would then look like:

   class MyExpression(Expression):
       def __init__(self):
           Expression.__init__(self, element, degree, value_shape)
       def eval(self, values, x):
           values[0] = 1
           values[1] = 1

Where element, degree, and value_shapes are all optional. All the logic of 
what element, should be used is concealed in dolfin.Expression.__init__

With this solution we split all the logic that goes into a compiled Expression 
from what goes into a user defined Python Expression. One of the main problems 
with the present design is that they are too intermingled.

Johan

> Johan
> 
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