UFL Specification and User Manual 0.3 - FEniCS Project

UFL Specification and User Manual 0.3Martin S. Alnæs, Anders Loggclass FunctionReplacer(Transformer):def __init__(self):Transformer.__init__(self)expr = Transformer.reuse_if_possibleterminal = Transformer.always_reusedef function(self, o):return FloatValue(3.14)element = FiniteElement("CG", triangle, 1)v = TestFunction(element)f = Function(element)a = f*vr = FunctionReplacer()b = r.visit(a)print bThe output of this code is the transformed expression b == 3.14*v. Thiscodealsodemonstrateshowtoreuseexistinghandlers. ThehandlerTransformer.reuse if poswill return the input object if the operands have not changed, and otherwisereconstruct a new instance of the same type but with the new transformedoperands. The handler Transformer.always reuse always reusesthe instance without recursing into its children, usually applied to terminals.To set these defaults with less code, inherit ReuseTransformer instead ofTransformer. This ensures that the parts of the expression tree that are notchanged by the transformation algorithms always reuse the same instances.We have already mentioned the difference between pre–traversal and post–traversal, and some times you need to combine the two. Transformer makesthis easy by checking the number of arguments to your handler functions tosee if they take transformed operands as input or not. If a handler functiondoes not take more than a single argument in addition to self, its childrenare not visited automatically, and the handler function must call visit onits operands itself.86