Subclassing Expression

Question

How to subclass Expression? The degree needs to be specified, but

from dolfin import Expression


class MyExpr(Expression):
    def __init__(self):
        super(MyExpr, self).__init__(degree=0)
        return

    def eval(self, value, x):
        value[0] = 1.0
        return


MyExpr()

doesn't do the trick.

TypeError: Expression needs an integer argument 'degree' if no 'element' is provided.

Answer 1

Why don't you pass degree straight to a cpp generated expression?

from dolfin import Expression


class MyExpr(Expression):
    def eval(self, value, x):
        value[0] = 1.0
        return


MyExpr(degree=0)

Answer 2

Hi, consider the following

from dolfin import Expression, plot, UnitIntervalMesh
from ufl.algorithms import estimate_total_polynomial_degree as get_degree

class MyExpr(Expression):
    def __init__(self, a, **kwargs):
        self.a = a

    def eval(self, value, x):
        value[0] = x[0] + self.a

f = MyExpr(10, degree=10)
assert get_degree(f) == 10

mesh = UnitIntervalMesh(100)
plot(f, mesh=mesh, interactive=True)

Calling the parent initializer is not way to go because of some metaclass related issues. See here for more.

Comments

Thanks Miro.

Problem occurs iff you define own __init__() in subclassed expression.