Cannot create domain-dependent expression

[AJ Lawrence]

Script from the FEniCS tutorial is broken in 2018. Particularly this snippet:

class Permeability(Expression):
    def __init__(self, markers, **kwargs):
        self.markers = markers
    def eval_cell(self, values, x, cell):
        if self.markers[cell.index] == 0:
            values[0] = 4*pi*1e-7 # vacuum
        elif self.markers[cell.index] == 1:
            values[0] = 1e-5      # iron (should really be 6.3e-3)
        else:
            values[0] = 1.26e-6   # copper

When instantiated, it throws a RecursionError on the third line into the Expression.py code.

[Markus Wegmann]

Hi there

We also ran into the same problem:

https://bitbucket.org/fenics-project/dolfin/src/f8cd6c0b2722bef82f1d429a9e838a4491b30154/python/dolfin/function/expression.py#lines-426

seems bogus, as __setattr__ and __getattr__ needs to be implemented carefully to prevent infinite recursion. See https://stackoverflow.com/questions/16237659/python-how-to-implement-getattr

The tutorial example ft11_magnetostatics.py also fails:

class Permeability(Expression):
    def init(self, **kwargs):
        print('Permeability created')
        sys.stdout.flush()

    def eval_cell(self, values, x, cell):
        print('values {}'.format(values))
        sys.stdout.flush()
        if markers[cell.index] == 0:
            values[0] = 4*pi*1e-7 # vacuum
        elif markers[cell.index] == 1:
            values[0] = 1e-5      # iron (should really be 6.3e-3)
        else:
            values[0] = 1.26e-6   # copper

mu = Permeability(degree=1) # FAILS!

Is reimplementing __setattr__ and __getattr__ required, or is our way of using the API wrong/outdated?

Best regard ;)

[Johannes Ring]

Use UserExpression instead (see for example the mixed poisson demo).

See also the changelog about deprecating subclassing of Expression in Python for the 2017.2.0 release.

[Markus Wegmann]

Thanks alot Johannes! Your answer is really appreciated.